Re- "Which is positive"....
If there is a diode inside the device, then, as Mike has said, it will be important to get things right. Isolate the device, test the resistance through it, and then reverse your leads, test again. Same readings? No internal diode.
At that point, the "negtive" side of the relay is whichever side you connect to your circuit's ground.
But wait a minute... just take a step back. Think about how the circuit will work, ignoring the diode. Imagine the circuit without the diode. Then put it in "the right way 'round" to avoid screwing things up... put it in so that when the voltage is applied, it will NOT go through the diode, thus it will go through the relay coil.