There is another way, you know, Lindy Loo...
UK frog and toad tadpoles apparently differ in taste. You pop one into your mouth for a moment, and then you know -- toads taste horrible. The taddie is not harmed if returned immediately to the water. Just do not inhale...
I have no idea if this really works, as I've never had the nerve to try it!
Where are you, milfik? Tadpoles in Europe should have frogged long since, unless they are exotic species (which of course should on no account be released).