Is a receipt enough? Won't you also need to show that you never withdrew the coins in order to prove you were actually stolen from?
You get a signed recepit for everything. deposits, withdraws, transfers. There is no action involving your bitcoins that doesn't include you signing something. If they can't produce a signed (by you and them) receipt showing you withdrew the funds, but you can show a receipt showing they have x balance, then you 'win' the argument.
If anyone is confused about how or why this actually works, I encourage you to read through the links above. It's a good read.