Veterinarian here.
Almost all vets would not bother checking your dog's penis or your bitch's vagina/vulva closely unless you specifically say there is something going on with that area and wanting your vet to have a genitalia examination. There is no reason for any vet to check out your doggo's genitalia unless you say something about it.
Even though let's say for instance you creampied your girl and she happens to require a vet visit shortly after, your semen would have liquified enough in her vagina tract and thinned out to appear like water/urine near the entrance of her vulva by 30 to 45minutes (speaking by my own experience). This does not raise any vet's suspicions; it will not raise my suspicion if I happen to see a wet and moist vulva on a bitch when I am consulting and examining her.
If you are still worried, wipe her vulva dry (in the vet clinic carpark) before you enter the clinic.
If your vet request to take samples from her vaginal tract/vulva for whatever reason, you can just politely decline. Your vet cannot force you to perform diagnostics without your consent/permission. You could take your girl in for the examination or vaginal sampling or diagnostics another day if you require her genitalia checked.
As a vet examining your male dog, I would not be able to know myself if you have recently made your boy ejaculate or penetrated you.