...you have a problem?
Give it a poke and if it feels squishy change it. And you have to be able to see the nappy anyway. If it set off a siren that you could hear through the child's clothes it would be useful. Or maybe a voice like some reversing lorry's have that says "Attention adults! I have urinated in, or have soiled my nappy! It's time for a change!"
Anybody bought them?