Only know if you penetrated or not. Don't let anxiety make you think you did if you didn't. But if you do believe you penetrated him without condom (and remember, only the head of the penis needs to be covered for protection), then that's a risk. But know this. First, sounds like super brief if it did happen, making risk smaller than the already super small and low risk it is. Unprotected anal sex is less than 1 percent chance of transmission. Point being, it's super low risk even if it did happen. Test at 28 days or later with a 4th generation test for confirmation.