How can we know if it was a baby tooth or an adult tooth that is loose?

Milk Tooth: Baby 's teething starts at 6 month finished at age 2 to3 years old. Permanent teething usually starts at age 6 .So you will know if that loose tooth is still milk tooth if not pulled out yet and replaced by permanent tooth.

Age + dentist: The age of the child can help you decide which it is. Front 4 top and bottom are replaced between 6-8yrs by permanent teeth. The baby canines and molars usually are not loose until 10-12yrs old. If you are concerned, go see a dentist and they can easily tell you and get xrays to make sure if needed.