There are a bunch of things to look out for. Improper machining is the most important. Valve seats should be inspected for proper width and depth. Spring seats should all be the same depth. To measure these two areas you need a spring installed heigth gage. It basically is a unit that sits where the spring would. This is a lot of work to check and a seller at a swap meet may not want to bother. Poor porting should also be looked for as well as pulled threads in all bolt holes. Basically everthing that serves a function needs to be inspected.