“If you have been tested and are positive for genital herpes but are not sure if you or your partner is the one that contracted it, how can you find out?”
There is no 100% way to find out how one or both of you were infected if either of you have had multiple partners. Your health care provider can take a detailed sexual health history from you and your partner, which may give you an idea of how/when one or both of you were infected.
However, if you have tested positive for herpes, the most important thing is to talk to your health care provider about protecting your health and getting more information on the ways sexually transmitted infections can be prevented.
Hope this helps.