1. Ask how many followers they have.
2. Do this.
3. Call them out for unfollowing you.
It's not like you're going to get a satisfying explanation, unless you really need to hear "I couldn't take one more whiny post about how your dad's cancer is affecting your diet."
4. Ask how they know your mutual friends.
5. Use Internet speak.
6. Only talk about the Internet.
7. Refer to them only by their handles.
8. Bring up embarrassing life events you only know about from reading their feeds.
It's one thing to talk about it online, and it's another to admit it in person.
9. Tell them they don't look like their avatars.
10. Assume they're DTF.
A flirty Internet persona could be just that: a persona. Respect people's boundaries, and don't think a bawdy back-and-forth means you're definitely getting laid.