Don't go off of glances or staring. Some people are just awkward that way.
If they take the initiative to text you first, if they make a point of coming up to you for chit chat, if they notice things like new shoes or a haircut they're more into you than a casual friend or acquaintance would be.
When texting or during small talk if they get cozy, they're probably interested in something more but want to read you before coming out and saying it.
Most men are so transparent and direct. If they're not making it clear, they're either not that into or they're holding back feelings because they're seeing someone else or they know you're already taken.