Comparing two retina iMacs

[mgb1120]

Comparing two retina iMacs, received one with a bad screen, got two in my possession, how do I compare screens without making an account on the replacement or is there another option? So I know which one to keep and one to send back

 

[hfg]

Boot from an external Thunderbolt or USB3 drive.

 

[rainydays]

Just make an account on the replacement and reinstall the OS if you decide not to keep it.
Actually you really don't need to reinstall either since they will do a reset if you send it back.