Most men will start a conversation with someone they are interested in. They also smile more frequently, offer to help a woman do things, flirt or use sexual innuendo, and the shy ones just keep "turning up" everywhere you are "coincidentally".
However what remains most important is not whether or not he is interested in you BUT are you interested in him?
A lot of today's women are lost when it comes to signaling a man using "subtle queues" to indicate his approach would be "welcome". The combination of being accused of sexual harassment, stalking, and the always present fear of rejction or the assumption the woman is already involved with someone else keeps a lot of men at bay.
Throughout the animal world the male approaches the female after she gives him the green light. Some males are naturally agressive but the majority look for some kind of "sign" whether it be eye contact, a lingering smile, looking at him then looking away and looking back at him....etc and other countless ways.
How about a toast to resurrecting the lost art of flirting? :-)