How to check device id of iPhone simulator?

As so often, there is a Xcode terminal tool for that (part of the Xcode tools) see xcrun manpage

xcrun simctl list | egrep '(Booted)'

lists only all booted (could be more than one) Simulators (remove | egrep '(Booted)' to see them all).

UIID results like

 iPhone 6 Plus (AAAABD40-9DE6-44B7-A4EA-B34ABCDEFCA6) (Booted)

you can then lookup a folder in ~/Library/Developer/CoreSimulator/Deviceson your Mac and find all the "belongings" of that particular 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

Try this

instruments -s devices