If you ever meet someone for the first time and you aren't exactly sure what their ethnicity and race is, it is perfectly fine if you politely ask them!

You know what's not perfectly fine?

Continuing to insist that you're right about their race after they tell you what they are by coming up with dozens of insulting theories, especially a bunch of ones centered around the idea that the person's mother had an affair with another man to try to convince that person that they are at least mixed.

Another huge no-no is not only describing the multiple ways that the person's mother could have had an affair in extremely graphic details, but also flirting with that person by using highly suggestive pickup lines while doing so. Especially if said person has asked you to please stop many times and is clearly very uncomfortable.

So unless you want to become "that creep" in a horror story that that person will be telling their friends for years to come, don't do that.