way to check a toddler's diaper for poop and/or pee?

see and smell , when you change it , you will again see and smell

take a peek....................

You can smell the poop of course, make a mental mote to check their diapers to see if they have peed after they have had a bottle or a drink of water, an hour or so. I used to make random checks if I did not see the diaper sag from being wet .

https://cdn.ebaumsworld.com/picture/RyannWeller/5....

You can generally smell it, if you pay attention. Urine and feces both are extremely malodorous.

You will know it yourself, just note, what time after eating or drinking they poop/ pee.