Welcome to the forum, I've moved the thread to an area where hopefully someone else can also help provide guidance.
The first thing you would need to know is the type of phone. Without knowing that I don't know of a way to know what bands it operates on. Just because a phone is unlocked doesn't necessarily mean it has the hardware required to operate on any US carrier's frequency.
If there are any brand names or serial numbers (under the battery maybe) I would start with those and some google searches to try and figure out what type of device it is.
"Is life so dear, or peace so sweet, as to be purchased at the price of chains and slavery? Forbid it, Almighty God! I know not what course others may take; but as for me, give me liberty or give me death!"
-Patrick Henry, March 23, 1775