Most diapers do have a wetness indicators on them now to alert the parents when the diaper is wet enough to need changed.
However if the diaper doesn't have wetness indicators on them then you can just feel the diaper with your hands and see if it's squishy enough to need changed.
If the diaper is just a little wet then it won't be very squishy however the diaper when wet enough will feel squishy in your hands.
As a general guideline a baby or toddlers diaper should be changed every 2 to 3 hours during the day and as the baby gets older they tend to wet less often and more on a set schedule.
A baby older than 1 year old may only need a diaper change every 2 to 3 hours while a newborn or baby under 1 year old would need a diaper change every 1 hour to 1 hour and 30 minutes.
Newborns usually go through 10 to 12 diapers per day while babies older than 1 year old usually go through 5 to 7 diapers per day.
Just check the baby often enough and if the diaper is squishy then it will be easy to detect with your hands.
When a baby has a pooped diaper then you'll usually be able to tell by the smell alone and you'll need to change the poop diaper right away to prevent a rash.