In Armadillidiidae the tails (uropods) are shorter than the last abdominal segment. (1)
Most other families can not roll into a ball and have tails longer than the last abdominal segment.
In the following image Porcellio scaber is in the family Porcellionidae and Armadillidium nasatum is in the family Armadillidiidae. Note the length of the tails in each.
adult image of Armadillidium nasatum
and photos of 2 other species not in Armadillidiidae (Scott Baird, Wright State U., Ohio)
and list of worldwide genera (ITIS)