"You're Polish if you think that about yourself....
If you are convinced that you're Polish and if you familiar with Polish culture and history. Could be difficulty without working knowledge of the Polish language but not impossible.
According to this lunatic ethnicity is a matter of belief
Ethnicity is an entirely different matter MR Anthropologist. lol!
A Polish nation had been created by the Polish culture but include different ethnicities. Nobles or better still szlachta {Read - shlahta}( as this word better convey meaning than the English word nobles which implies exclusivity and sometimes aristocracy) adhered to the one dominant Catholic-Polish culture while being ethically diverse, including Ruthenianians all kind and shapes, Russian (better still Muscovites), Germans, Poles, Jews, Scots, French, Armenians, Tatars and so on....mixed with freedom.
Take for example Belarusian and Ukrainian they have less history, culture and factual claims to their own nationhood than nations like, Burgundy, Lotharingia, Normandy or Aquitania. It wasn't written in the stars that those nations each one with more factual claim to nationhood than 1000 states like U or B has to be French. Bah 200 years ago only minority of the populace in today France actually spoke French.
Nationalism and freedoms for peasants changed masses into French or what not.
Poland didn't exist in the whole 19th century.
To the point - modern Polish nationalism went to catholic peasants those who spoken local version of the Polish lingo and taught them to be Polish. Hence your 'ethnical' Poles.
To be a Pole is much more than that.
Europeans are indigenous European people with many tribes that mixed and enweaved for millennia. Funny that Germans being genetically the most mixed modern nation has that myth about one tribe.
Same with Italy, their unification is based on a pure cruel conquest of the south by the north (Piedmont) that almost wiped out their distinctive although dormant national identity and left them with mafia.