Contacted STI?

If a man and woman don't have sex (penetration) but he lightly rubs his penis on the outside side of her vagina very briefly (without a condom) and never cums, how likely is it you think that he could have passed on an STI to her? I know some STIs can be passed from skin to skin contact, but how likely you think that would be in this scenerio?
When would be a good time to get tested for STIs to be sure?