how could they tell...

Well they could not tell 100%, but dark hair and dark eyes were a bit of a hint.

Jews and Poles lived largely separate lives and were socialized very differently.

Biggest, most obvious difference: the majority of Jews used Yiddish as a first language, a minority Hebrew, and then, very few, used Polish as a first language.

Polish and Yiddish aren't even related. Yiddish is a Germanic language, Polish is Slavic.

Jews' ancestors largely come from the Middle East -- recently published studies support this. Mediterranean people tend to look different from Northern Europeans like Poles.

And socialization was different. Body language was different. Clothing was different. Grooming was different.

Males could be stopped and asked to drop their pants.

Children could be asked to say prayers.

Etc etc etc.

The Nazis stole synoguge lists, checked identity cards, forced Jews to wear yellow stars on their clothing to identify them, some Jewish groups had distinctive dress and hairstyles which easily identified them, at that time few non-Jews were circumsised, in many places they did look different from the rest of the populace (Judaism can be a religion, ethnicity or both), Jews tended to live in Jewish villages and neighborhoods, they ate different foods, had different holidays and custums, they were forced to register, then they were forced into Ghettos completely isolated from non-Jewish residents. The Nazis were methodical and efficent in identifing and pursecuting minority groups.