To check if a diaper is full you can feel the diaper on the outside and see if it's swollen up and squishy.
If the diaper is really swollen up and squishy then it's full of pee and should be changed.
You can also check the back of the diaper for poop by pulling the back of the diaper out and looking into the diaper to see if there's poop.
If the diaper is poopy then it should be changed right away as soon as you notice it.
If the diaper is full of pee then it should also be changed.
You don't have to change a wet diaper right away if it's not full as disposable diapers are so good now at absorbing the pee that the baby or toddler doesn't feel the wetness.
If using cloth diapers they should be changed when you notice they are wet.
If the baby or toddler is wearing disposable diapers you don't have to change the diaper after every pee and you can wait until the diaper is close to full of pee or is full of pee before changing it.
However the diaper still should be changed at least every 3 to 4 hours regardless if the diaper is really full as the urine can turn to ammonia and start burning the skin.
Some disposable diapers have the wetness indicator on them, but even then just because the wetness indicator indicates wetness you don't have to change the diaper right away.
I change my kids diapers every 2 to 4 hours for pee depending on how much they pee, but when they poop I change it as soon as I notice and can change it.