The way to tell impeller working or not is to hook up muffs and fire up the boat in the driveway.
A new to you boat needs all maintenance items gone through. You must assume none of them were done as the previous owner (PO around here) got tired of his boat and stopped using or maintaining it. Get yourself the Mercruiser manual for that model drive. And go through the maintenance. Then hook up muffs and test that everything works before you have to walk back when something doesn't.
Congrats on the new tub!