Most gaymen have what is known as a gaydar sense, we can look at another man and pick up a vibe which says - he is a brother. It is very strong in some people and not so strong in others and sometimes it is completely incorrect.
Unless you are in a gay environment, the chances of learning if someone who has caught your eye is gay, will be very difficult without asking them. Tell tale signs to help you assess their status can be clothing, body image, the way they sneak peaks at other men, the way they stand/sit/walk. To show interest, simply try to make eye contact, if you get caught and they smile, get up and go talk to them (not necessarily asking are you gay) ...if they turn away and ignore your passing glances - give up, they are either not gay or not interested in you.