Can you recognise the nationality of foreign Polish speakers by their accent?

No way! I dont believe you. Do you really cant say: Aj łant tu hajer jor karr? ;D

I've tried, I always end up sounding like a Paki lol

Which problems are those?

misprounance of "sz" for instance

I can usually recognize a Polish speaker when they say, Good Morning (Goot Morrrning.)

I can do it with my ears rolled up but whatever for ?

by the way wildrover cut your hair !

Lol.
They cannot say the "i" in "is". No matter how much you try to teach them, they wil always say "ees", also the endings "ing" are always said "eenk", for example "speakink". And the list goes on and on. You can tell who is Polish who is German or who is Hindu by the way they try to speak English. It's funny. Lol

^ Technically they're not incorrect, since English is one of the loosest phonetic languages around. However, most Poles have serious problems with simple English vowel sounds. For some reason they never learn the difference from Polish vowels.

Well I pronounce "is" as "ys"... that's how I hear it. "Ing" as "yn" rarely pronounce "k" at the end

Lol, sounds similar to Zsa Zsa Gabor's accent. Instead of saying darling, she says darlink.

They also pronounce "ę" at the end of words like "a". Dziękujaa ;D

Don't Silesians do something like that too?

The old "ostpreussisch" accent of elderly surviving ethnic German speakers from around fomer Koenigsberg etc.. have markedly trilled 'r'-sounds when speaking German. Often times, their family names too are pure Polish, Krawitz, Kaminsky, Wapniewski etc... and frequently sound like Poles, not Germans, when they speak.

Still others such as the now-demonized Guenter Grass from former Danzig (now Gdańsk), sound one-hundred percent German when speaking their mother tongue!