2000 Ford Excursion Engine won't start

200 ford excursion. When trying to start I get a thump and nothing. I have replaced batteries several times thinking that was it. This is an intermittent problem and is usually gotten around by jumping the battery or waiting some time.
I suspect the starter since the alternator checks out good.
Can you tellme how to be sure it is the starter before I go changing that too on a vehicle with this many miles.

Thanks,
Eric