How to check device id of iPhone simulator?

EDIT: See other answers for the newer ways to view UDIDs for CoreSimulator-based simulators.

1. instruments -s devices
2. xcrun simctl list
3. From Xcode: Window -> Devices and Simulators -> Simulators.
   The Identifier value is the UDID.

From 2011:

Is the iPhone Simulator UDID unique for each installed instance?

This answer should be what you want. Look at System Profiler on your mac and the id is there. I just tested on my machine and the IDs match.

This is the exact Terminal command you can enter to view it:

system_profiler SPHardwareDataType