Guys, how is a girl supposed to know whether you're joking or really are interested?

[faithehopeelovee]

I'm just curious of how you would show a girl that you actually like or, vs. just joking around and being nice. There is this one guy and I just can't tell whether he is interested in me, or is just joking and being nice. So, how would you show a girl that you're actually interested? Girls, you can help too! Any opinions are helpful! Thanks in advance!