There is a lot you can inspect! Is it an old rig, which has been in for service and has been repaired? --> API 4G. I found the CAODC (Canadian Association of Drilling Contractors) guidelines very useful for the structural elements of work-over rigs. The main emphasis on the rig structure will be welding inspection. Checking a good sample of welds, requesting paint to be stripped from welds where you want to take a closer look, MPI, dye penetrant, etc. Beyond this, there are a lot of specialist areas to look at and really a specialist should do it. I recommend you read all the API's on drilling equipment. To get a fitness for purpose you will need at least 10 days on the job. A rig is an entire industrial plant in itself. You have electrical, pressure equipment, structure, liftings, OSHA, rotating and all that under heavy use conditions all there in one spot.
If you have specific questions, post them and we will answer.
