I'm very late to this party, but:

Correctly ascertaining others' internal state and responding accordingly is a NEWT-level social skill. It is (at least usually) easier to ascertain your own internal state, specifically as it relates to the particular behaviors of the maybe-flirter, and respond accordingly. Here is how this breaks down for me:

"They might be flirting and I like it":

-> And they are flirting: continue whatever I was doing, remembering that flirting is no guarantee of any particular outcome

-> And they are not: same (such should be the conviction that flirting is no guarantee of any particular outcome.)

"They might be flirting and I don't like it":

-> And they are flirting: Excuse myself from the situation; ask them to modify behavior if it recurs (or avoid them)

-> And they are not flirting: Their take on acceptable platonic interaction makes me uncomfortable, so again excuse/ask/avoid.

So, conveniently, it doesn't matter!

Of course, it's generally a fine idea to just ask, too, remembering that the given response may not be completely reliable. :)