Hi,
On the client side:
If you are using Firefox, I'm not aware of a way to check.
On the server side, there isn't much you can do without issuing unique turn server ids and checking the logs of the turn server afterwards because the communication negotiation is handled on the client side.
If two peers aren't separated by firewalls or routers implementing NAT, they will always use peer-to-peer. Otherwise, you can try simply not including any stun servers in your ice configuration.
Eric.