you only want to go undercover with sufficient evidence. So you basically get a lot of leads from whatever filtering program you have, and then have to call the providers and ask them for records of such-and-such a person. Or you send them a list of the dates and they have to confirm. For places like eye glasses or massage, they may have to get the claimant to verify who, exactly, is the RMT or optician or whatever. Lots of spreadsheets, cross referencing, working with the company HR, etc.
My friend that does that work hates it. Rarely does she have to go out of the office and do anything undercover.