" i ask them how many months out of the year the child lives with them and they tell me 50/50"

I tell my clients that neither one of them can claim the child.  I then tell them I need to know which parent the child slept with every night of the year

"can they determine who is custodial parent?" 

That is tax fraud and if my clients want to do this I ask them to find a different preparer.

You need to review the rules so you are strongly familiar with them and don't get into doing something that can get you big fines!

Dusty