Signs that someone is crushing from afar

There are no obvious signs, and you will never no for sure unless he tells you directly. That said, here are my observations:

He looks at you more than normal (i.e. most straight guys don't want to give a wrong impression) and pays special attention to what you're doing. Some guys will try to be discreet and only look at you when you're not looking at them. If you're good at observing with your peripheral vision, you might see that happening. In those cases, you will often see him turn away when you look at him. This can happen multiple times, which is a good way to check the first time wasn't a fluke. You might try smiling at him and see if he smiles back (at least a sign he's friendly).

Ultimately, you will need to actually talk to him to get a better feeling for what he's thinking.