There is no sure way. Unless you have the guts to confess.
Which you'll have to do at one point or another. Unless you stay silent forever.
I'm assuming you want signs. Well, if a guy talks to you a lot or makes you feel special. If a guy spends time hanging with you, more so than with others. Those are ways. However, since guys are dense, it can also be simple signs of friendship.
We all know that it's not easy to confess. We face rejection, we face hurt, we face pain we could've diverted. It's Russin Roulette...though you do get a little bit of an advantage in the fact that you can (somewhat) weigh your chances.
In the end, it all comes down to cooperation between your brain and heart. Heart to what your emotions naturally tell you (avoid the bias if possible, it's very annoying and causes 99% of heartbreak). Brain to making sure your heart doesn't get chased into dreams of fairytales or too overly pessimistic. That usually gives you the grounds to confess.
Or you can wait. Waiting it out often sorts out emotions. If you're not 100% sure you like someone, waiting might be the best option. Waiting allows you to think. And while love (IMO) is not in the least bit scientific or logical, thinking it through is necessary.
If you really cannot decide, then let time tell. Time defines relationships of any sort and it'll become clearer as time passes.