Inspect a crane for purchase? Or before putting it into operation?
Operating: If you've done your OSHA training, then it should be a "qualified person". They love that term, but that's loosely defined. Basically do they understand cranes? But, an NCCCO (National Commission for the Certification of Crane Operators) Crane Inspector certification is highly recommended for operation. Could vary by state what's actually required. Regardless, it should be looked over regularly by a qualified person before every use.
Purchasing: Same applies but the bar is maybe even lower. Most equipment inspectors have knowledge that transfers. They'll get photos of all moving parts and can likely spot things that don't look right. Ideally you'll get an operating video, but that might not be possible if it's a dis-assembled tower crane for example.
To be safe getting an NCCCO inspector would probably be your best bet either way. They've got directories available you can search by location.