Its trickey. Check plug look in cylinder with a pen light . No stripped bolt heads , all bolts similar size. Owner should know specifics about engine. See if crank moves side to side or up and down. Should be semi clean , the general looks of the surrounding area or shop, trailer or other racing equipment. may be as good a indicator as anything .
Start if possible turn it over with puller listen for noise and compression. Oil left in since last race? How long since ran.
Flatheads need attention sooner then clone so if it's a full season of running it may need work(valves and guides) . Google the person or check their past posts on bobs . Lots of good used engines , a few not so good . Acting shakey probably are.