He would more than likely be drooling heavily. They usually grab them and the frog will excrete the poison which usually makes the dog drop it but does make them salivate heavily. I would watch him closely and should he start vomiting and acting listless then a vet visit ASAP is a good idea but I would assume he didn't actually get hold of the frog as you suspected.
Let me know if you have any other questions
Good Luck &
Best Fishes