There used to be helpfull pics like this in the gallery but they have all gone now
Like gtir350 said it's the one with a tab poking out for a fast-on terminal (spade connector).
I have a sensor in my desk draw now, but the fecking thing is locked and my key is at home. I'll go ask the receptionist if she has a spare. Do you want me to send you a pic message of it? (if i can get my draw open)