How do you tell if your girlfriend kissed someone else?

You can't tell if she kissed someone else enless someone told you she did or if you saw it. If she starts loosing interest in you majorly then sit down and talk about why, if she is defense she may be cheating or thinks she simply likes someone else, she might be going through a rough time. However, if there's lots of trust in your relationship and she really loves you you shouldn't have to worry.