Look deep into his eyes. When you're speaking to him, does he gaze attentively making eye contact? Men instinctively cannot resist looking at what their mind and body desire. As you observe him watching you, are his eyes 'body checking' you? If his eyes move from yours down to your mouth and then down your body and then back up to your chin, well, you've got him hooked!
Check out his moves. Subconsciously we mirror or mimic body movements of people we are interested in. If you tap your fingers, does he?
We all like to look our best, particularly for someone we want to know better. In men, preening rituals include adjustments of clothing (tie, collar, shirt), dusting imaginary dust from their shoulder and running fingers through their hair.
Does he communicate availability to you in his body language? When sitting, does he cross his legs away from you or towards you? If towards you, he's trusting you with his manhood, his most vulnerable area. If he's opening his body towards you, he might very well be telling you without words that he's open to much, much more.
Look out for a man with his hands on his hips or one who hooks his thumbs in beltloops. This is done to make him appear bigger and more virile and indicates he is sexually available.
When speaking, does he focus on his achievements or possessions? He might very well be trying to market himself as your potential suitor. Take note of the attention he pays to asking questions about your life, hopes and interests.
When looking at his language, it's worth noticing whether or not he's mimicking or echoing your words or phrases. He may be doing this to try and emphasize your connection on a subconscious level.
A man who is interested will touch the object of their desire subconsciously, just as a woman will. Whether it's brushing his hand on your arm or removing a piece of fluff from your coat, it's all happening to express his interest in you.