Evaluating similarity between two subsets

i have the following problem that i cant figure out.
i have a set ID's which i pseudo-randomly split into 2 subsets A and B. lets say for the sake of simplicity i did it twice only, so i have subsets A&B and A&B. the sizes in the different splittings are the same - not sizes of A and B, but sizes of A and A, and of B and B.
what i need to know is if there is a standard way of telling how different those distributions are. i just want to make sure that subset A is not different from A or B by just a couple of ID's (sizes of A and B differ by 1 only).