They are easy to spot. The best thing is to keep quiet and not let them know you know than when all is quiet and forgotten about (on their part) then is the time to do something sneaky and nasty. Normally those type of people think they are above anybody who is smiling to their faces to remember they were ### on by them. So do what you do in quiet and you won't get caught. May be a week or may be a year, just bide your time, you will get their trust again. just don't be like the "backstabber" and do it first.