Is there a way that I can tell if my child has scabies?

Small bumps. First, determine if you have been exposed. A positive exposure makes it more likely. There are usually little bumps in linear distribution on flanks and arms/ legs, or often on the hands, knuckles and webbing mainly. It's very itchy. If you suspect this, show your doc to confirm and treat.
Scabies. Fine rash on the body, scratching more at night. Also there will be rash in the webs of the fingers, lot of itching- think about scabies and verify with your doc.