Assuming it cannot be started
and run. BRBR
When I was in the market for a used motor some years ago, I took along with me
a saw horse in the trunk of my car. Every motor I considered was then set up
on the horse, a hose connected with ear muffs, or I asked the owner for a large
garbage can to fill with water. It's an easy thing to set up the engine and
run it -- which I would recommend before buying any used outboard.

Another possibility is to take the motor (and the current owner) to any
outboard repair shop and ask them to run it for you in their test tank. They
may charge you for it, but it would be a nominal fee well worth it.