I'm not a frog expert by any means, but if you put crickets in your frog's tank and they disappear, I think you can safely assume that the frog ate them. If the frog wasn't eating when you dangled crickets in front of it, it most likely just doesn't want to eat from your hand or whatever. Like everyone else mentioned, don't kill the crickets because frogs definitely want live, moving food to chase. If you wanna be sure, you can always put a cricket in and then watch for a while if you have time.