Oral sex exposes you to gonorrhea, syphilis and herpes. As you did only a lick, the risk is close to zero. Hand to genital rub doesn't transmit any STDs. As you licked the other guy's glans, you might have been exposed to sexual secretions, so it would be interesting to test for the diseases I talked before. You can do it as follows:
-3 weeks after the exposure, for gonorrhea.
-6 weeks after the exposure, for syphilis.
-3 months after the exposure, for herpes.