there is a one way u have to check the rotor if the
commutator and brush assembly present in dc motor but not in
AC motor .
but in size dc motor size quit large compare then Ac motor
and also usually Ac motor has outer cooling fins and blower
which is fixed on shaft in case of dc motor forced air
cooled inside the motor in-order to dissipate the heat .
Ac Motors Body construction is slotted where In Dc motors construction is plain.reasons are given below.
In ac motors armature is static therefor more current for cooling purpose fins are required,thats why on motor body slots are given.
In case of DC Motors Armature is rotating therefor heat produced due to armature current.there for cooling ventilation
is required.and field is static,and it takes very small amount of current so thre is no cooling requirement for that.