WebRTC: force peers to use TURN server

Question

I have a webrtc application, it works fine, but for testing purposes, I need to test if my TURN server works, but because both the testing devices are within the same network, I

Answer 1

just adding this to close the question,

function onIceCandidate(event, targetSessionID, targetUserName) {
    if (event.candidate) {
    var candidate = event.candidate.candidate;
    if(candidate.indexOf("relay")<0){ // if no relay address is found, assuming it means no TURN server
        return;
    }...

The above code works, checked to wireshark,

after adding the if(candidate.indexOf("relay")<0) condition, communication takes place only through TURN server, if server is not present/ incorrect details, connection state get's struck at new

Edit: like cullen has said in his answer, according to w3 webrtc, passing relay as iceTransportPolicy should work, but I haven't checked if it is implemented in Firefox and Chrome yet...