I always ask for copy of vet records. Many times these don't include if dog is on flea/tick preventative and/or annual fecal test results.
I do make sure pets coming into my home are fully vaccinated. My dogs are and I expect that of clients for the health and welfare of all the pets. Boarding facilities that are not Rover related require this and I would not expect lesser standards just because it is in a private setting.
You should ask all the questions you need to ask to make yourself feel comfortable with the situation. Something you might consider is drafting a vet release for your clients to sign prior to the dog's stay. Mine allows me to seek and authorize treatment for their dog, includes a payment clause, and also allows me to contact the vet regarding routine medical treatments so I can ensure the dog is getting regular vaccines and exams. Personally I don't ask about proof of flea/tick preventative. If I spotted any issues with the dog at the M&G I'd require them to be resolved before accepting the stay or decline it altogether.
I think you're doing the right think just by asking about all this to protect yourself, your family (including fur-kids), home, and know of the guest' status. If you have the vet information, you could probably check fecal results not on paperwork. With flea OTCs, I ask which one & take their word.