If they do a poor job by doing too little maintenance, your renter will let you know about it.
If they do a poor job by doing unneeded or unimportant maintenance, and/or charging you too much for the tasks they're doing, your renter will be perfectly happy... so how will you know?
Granted, it's important not to obsess about this stuff -- if they're cheating you just a little bit, it's not worth worrying about. But if you don't know what you're doing, they may be able to cheat you pretty seriously, and you won't know.