have you and your partner ever been tested for herpes before to know who has what?
grace
If you don't recall a history of cold sores then this could possibly be a cold sore or irritation, zits, etc. Cold sores are typically painful blister(s) on the lips, and since there is no pain, it's unlikely this is herpes. Most adults have cold sores (HSV-1), so if you're that curious, you can get tested with an Herpes Select IgG type specific (HSV-1) test. A culture or swab would be more reliable, and it's possible you've had oral HSV-1 for years since only about a third of those that have it are symptomatic.
You're only contagious from your mouth if this is in fact a cold sore, so you should avoid oral sex. You can have as much regular sex as you want.