How can you tell if a surname is Polish or Polonized, and... what the heck am I, ethnically?

I've been trying to uncover the Slavic half of my roots. Previously, I was under the impression that I was half Polish, and that side of my family of origin was too, but after submitting my spit sample to 23andMe it's unclear just how much of my roots are actually Polish and how much is actually some other Slavic (or even potentially some other non-Slavic!) ethnic group that Poland went in and Polonized, since political borders have fluctuated rather vigorously in the regions from which I have records of known ancestors' birth/residence. I'm a PhD student in molecular bio with a specialty in computational genetics so I understand the limitations of this sort of testing very well, but my specialty is decidedly not in human genetics or ancestral admixture.

Genetic information:
1) 23andMe has found some real interesting things. At the default (50%) confidence level, I am listed as 47.4% Eastern European, with Poland highlighted (5 dots under the 'scientific details' section) and Ukraine also somewhat (2 'dots'), as well as 1.3% Balkan (basically the former Yugoslavia, Romania, Moldova, Bulgaria, and Greece). At the 60% confidence level, 23andMe is still confident that I am 45.1% Eastern European and 0.3% Balkan.

2) When I chucked my raw data through the admixture calculators at GEDmatch.com and sent it into DNA.land just for giggles, I got some odd mixed results: not actually much Polish to speak of, especially from the calculators that actually HAD Polish samples.

- DNA.land (kind of cruddy and doesn't have Polish pops) thinks my Slavic ancestry is a mixture of Belarusian, Lithuanian, Russian, and Ukrainian (and for some reason it thinks I've got a chunk of Uralic in there - Mordovian, Estonian, and Finnish pop up)

- Eurogenes K13 (has Polish pops) thinks I'm Lithuanian, Belarusian, and Russian
- Eurogenes EUtest v2 K15 (has Polish pops) thinks I'm Ukrainian (specifically, the Ukrainians around Lviv, which isn't entirely insensible given the Galician ancestry)

- Eurogenes EUtest (does not have Polish pops) thinks I'm Ukrainian, Lithuanian, Northwest Russian, and Belarusian
- MDLP World-22 (has Polish pops) thinks I'm a lot more Belarusian, Lithuanian, Russian, and Ukrainian than Polish, though it recognizes the Polish apparently
- MDLP K23b (has Polish pops) thinks I'm West/South Belarusian, Kashubian (Polish, I know), Lithuanian, generic Baltic, Latvian, and Russian
- MDLP K16 Modern (has Polish pops) thinks I'm Latvian, Belarusian (particularly from around Gomel, which is weird because that's Eastern Belarus), and Lithuanian, and maybe slightly concedes that I'm part Polish (with a particular focus on the Sorbs).

Towns and surnames in question:
1) On one side of the Slavic part of my ancestry...
- I have actual living relatives in Pisz, in northeastern Poland (north of Lomza, northwest of Bialystok)
- I am pretty sure one of my great-grandfathers was from Bialystok
- I've heard Lomza tossed around a few times
- The surname 'Sielawa' is found pretty heavily on this side, and I am aware it clusters mostly in the east of the country (it's the Polish word for the vendace, Coregonus alba, a kind of European whitefish that is found in the area. c.f. Belarusian 'cялява' and Lithuanian 'seliava')

- Others: Prusinowski (I'm aware this refers to Prussia), Duda (I'm aware this is common)
2) On the other side...
- One of my great-grandfathers was from Gorliczyna, in former Galicia (near Przeworsk/the Ukrainian border)
- The surname 'Burat' is found pretty heavily on this side, and I'm not positive at all this is Polish in origin
- Others: Taroski (pretty sure this is Anglicized), Bednarz (which I think clusters around Katowice and Opole)

Other surnames associated with people I've been listed as third to sixth cousin of on 23andMe, and so I share anything from a 2nd-great to 5th-great grandparent with them (and I don't know the names of any of these ancestors): Wyszkowski (Vyshkovsky?), Krutina (almost certainly Russian in origin), Dolinova, Staffova, Kalusek, Vitova (all Slovakian), Trojanowski, Czyzewski (may be Polonized Belarusian, especially because the surnames Hoskowicz and Pietkiewicz are associated with it, and I've heard -owicz and -ewicz are especially likely to be Belarusian), Dzwonkowski, Kowalewski (Kovalevsky?), Gabrych, Generalski, Janczukiewicz, Warminski, Jankoski, Nowakowski (Novakovsky?), Jorsz, Jursz, Zacek, Wysotki, Jablonski, Sendrowski, Krupski, Zilnicki, Pierzchanowski, Perkowski (Perkovsky?), Pent, Androv, Bakardjiev (definitely Bulgarian), Anuszewski (definitely Januszewski - I'm pretty sure that's Polonized Belarusian, Russian, or Ukrainian), Rajniak, Chudik, Benchurik (Ukrainian?), Strzelecki, Radulski, Anton, Wieszek, Terzic (definitely Serbian/Croatian), and Lewandoski.

Given this information, how can I tell, and what's the likelihood, of some of these surnames being from somewhere other than Poland? And what the heck am I if not all Polish, am I some nebulous mixture of Polish, Belarusian, Ukrainian, and Russian with a smattering of Baltic and Uralic?

Correction: EUtest has Polish populations.

Which country are you from? If you're from the U.S. you can obtain census records to see which language your immigrant ancestors claimed as their native tongue. Ship manifests and immigration records can sometimes assist with this as well. Alternatively, you can research your family tree to see which exact towns or villages they lived in. Once you've pinpointed them on the map, you can obtain an ethnographic map from a historical atlas (many of which are freely available online) to see which ethnicities lived in that area at that time. Some areas were multi-ethnic, sometimes making it difficult to determine which ethnicity your family belonged to...

To give you an example, I had ancestors who were ethnic Lithuanians living a few miles away from the modern day border with Poland, in a region that was historically occupied by both Poles and Lithuanians. I only know that they were Lithuanian because older generations in my family remember our immigrant forbearers identifying as Lithuanian and speaking Lithuanian (which is confirmed by their census and immigration records). Adding to this confusion is the fact that the language used to record vital records does not always reflect the ethnicity of the person documented. In the case of my Lithuanian ancestry, from circa 1800 to 1863 (or so) all the vital records were recorded in Polish, and the names were also Polonized! After 1863, all the records are in Russian, but the Polish equivalent of the names were still provided (likely because the region they were from had historically been part of the Kingdom of Poland).

I recommend reading up on the history of the Polish-Lithuanian Commonwealth--this territory included lands occupied by the ancestors of the Poles, Lithuanians, Ukrainians, and Belarusians (to name a few). Having a sense of the "big picture" will help you understand the overlapping ethnicities of this part of the world.

For locating genealogical records from Poland (and other areas formerly part of the Polish state) I highly recommend the following sites:

szukajwarchiwach.pl
genealogiawarchiwach.pl
genealodzy.pl
poznan-project.psnc.pl

Good luck!