I was told to redirect my question from /NoStupidQuestions

I am reading an article on the language ability of African Slaves in the 18th century and the author is referring to some sort of slave owners' gazette where they post about run-away or captured slaves.

Anyone, one quote had me thinking "Committed to the gaol (jail) of Lancaster county, a negro man named JAMES, who says that he is free and will not own (up to) having any master...he has past(sic) as a freeman for several months."

Now this is an ad from a writer in Pennsylvania, a free-state, preparing to send a man "down the river" literally. So, as there were free blacks in the North AND the South, how did they prove this? Yes, I know about 12 Years a Slave and he was captured by crooks specifically to be sold.

But what could that man, arrested by a civil authority, do to prove his freedom?