Best way to inquire/require guest dogs to have flea treatment?

I'd recommend that you mention in your profile that to protect the well being of all dogs in your home that all dogs must be on flea preventative, as well as anything else that is important to you (Vaccinations? Fecal/heartworm exam for parasites?) During the initial exchange of messages, you can confirm the dog's status in writing. At the meet & greet, you can easily ask what the dog takes and if will be due for any administration during their stay. I've found the pet-parents can immediately answer what the dog gets without hesitation and good eye contact, may need a moment to think if dog is due, and appear grateful you're thinking ahead about their stay. If someone isn't treating their dog, that will likely be obvious and then you can discuss if you're comfortable providing care (maybe at their home rather than yours? or if dog stays inside due to medical concerns / doesn't go out, maybe other steps are taken such as a regular flea comb.)

Ultimately, you are not required to meet or accept a dog who isn't a good match. That includes pet parents who do not treat their pets until there is an issue, which could blow up in your home. I realize prevention is just part of the solution as a flea can hitch a ride on a treated dog, but at least it helps mitigate the problem.

I've been thinking about this issue too! Fleas in my area are immune to most topical treatments, so even dogs that have been treated could still be infested! I'm thinking about asking about rabies and flea treatment during the initial message exchange or the M&G. I'm not sure how an owner could prove to me that their dog is treated, but am hoping just bringing it up will remind them? I also want to start treating my yard and even the carpeting in my house every few weeks as a precaution. Does anyone have recommendations for the best pet-safe products?