Clearly, animal shelters inspect dogs before taking them in. I also imagine there being quarantine areas for newcomers.
How do smaller for profit doggie hotels/day cares or grooming shops deal with this? Some of these can be pretty posh and I don't imagine well off customers would tolerate getting back a flea infested dog. However, if there are multiple dogs on the premises, one of them could easily bring in such pests. Or is the detection rate of an experienced handler good enough to rule out such problems?
I know that especially dogs from wealthy families are unlikely to have such issues, because they will likely be given preventive treatment (collars, powder etc). I just want to know how business owners protect themselves from an irresponsible minority of dog owners.