How or where can I go to find out the real last name and where could i go to verify documents.?
find great-grandpa's naturalization papers.
check arrivals at Ellis Island. This will help to determine if the name was changed on entry to the usa, or later
find him on the Ship's Manifest and then check next of kin. next of kin may or may not have the original name, but it's worth a try. next of kin could be just a friend though
what were your great-grandparent's first names ? Did they change them too ? Or did they keep the original form.
The best way forward is to have your uncle write/spell the name. Then you will have something to work with.
The spelling you gave doesn't exist, but there are close matches.
Find out which town or city they came from... then we can do a name search.
In other words, we can help you, but we need more specific information.
Given that this is your great-grand parents, please read up on the history of Poland before 1918. You might find that with border changes they actually came from an area that is not now Poland.