1) They tell you.
2) They show up around you more than they did before.
3) They ask a lot of questions about what you like and don't like.
4) They "accidentally" bump into you (more than once).
5) They yawn right after you do ("The Yawn Test").
I want to elaborate a bit on "The Yawn Test."
When we were in 8th grade, one of my friends and I came up with this great idea for figuring out which girls liked us. We called it "The Yawn Test."
Here's how it worked:
Yawns are contagious...when you see someone yawning, then you yawn right after they do. So right after you yawn, look around (subtly) to see if the person you hope is yawning is actually yawning. If they are, then it means they were checking you out!
But consider yourselves warned:
This is not an exact science.
One day this friend of mine sat down at the lunch table. He was super-excited. He said that he had just performed the test and couldn't believe what happened. The girl that every one of us wanted to slow dance with at a middle school dance had yawned right after he did! And you know what that means: it was obvious that she was into him!
So he strutted over to her lunch table and asked her out.
And what do you think she said?
She said, "No."
So it doesn't work every time. If the results are positive, you can be sure that it's not a false-positive if and only if you've noticed some of the other signs happening too!
In other words, don't just rely on The Yawn Test. Use it as more of like a final confirmation after you've noticed that the other person is showing up around you and talking to you a bit more than before.
Happy Valentine's Day...and good luck!
For more pointers, see the helpful article (with illustrations) over at Wikihow.com
What You Really Want in a Boyfriend or Girlfriend