It's not always possible to tell if someone is wearing a diaper especially if the diaper is a thin discrete diaper that the person is wearing.
But sometimes you can tell especially if the diaper is a thicker diaper and you may notice a diaper bulge through the front or in the back of the pants.
When I wear my blue jeans and am wearing a thick diaper and it starts to get wet and sags you can see both a bulge in the front of my pants and also see the outline of a diaper and a bit of a bulge in the back of my pants.
The back of my pants will usually be pushed outwards a bit and you can see like a V shape or maybe a U Shape in the back of my pants as the diaper is sagging in my pants.
I don't try to hide my diapers so if someone sees the diaper bulge or notices I'm wearing a diaper that is perfectly fine.
I wear diapers all the time now and use them for both pooping and peeing in and I no longer use the toilet at all.
You might also notice the waistband of a diaper as well.
But some people do cover up the diapers really good if they're not wanting to be seen wearing a diaper but if you saw me you'd really likely notice I was wearing a diaper especially if the diaper is wet.
For other people you'd have to look really close at their crotch or butt to see a diaper bulge or waistband of the diaper.
If you look closely at a diaper age kid wearing diapers under pants you can usually see a droopy diaper in their pants or see a diaper bulge in the front of their pants.