I'll give you my opinion on the matter, so it won't be scientific or anything. I've just been a guy and around them enough to know what they are like sometimes. Since everyone is different, I'm sure others can chime in too.
The first thing you'll notice from anyone that is interested and is a typical man is that they will show off to people they want to date. Even if they just want your attention, they tend to just show off. I'm pretty sure it's to show that they are special in some way and to try and capture attention that they don't feel they are getting. You see it all the time with boys being stuntmen on their bikes, or men in cars driving like morons when a woman may be around they like(though some guys just show off all the time regardless).
Men will try to talk about random things to you to try and hold your interest. Just something like "how is the weather" can be an indication, but sometimes they may just be being nice. At a store, for instance, a clerk may not like you if they ask you simple questions. But a guy coming up to you out of nowhere in public or not so public places to chat about random topics, especially things that involve what you both do for a living may be trying to see if you're compatible.
Some men are just blunt, so that's another sign. They will just come up and ask you on a date eventually most of the time, even if they're shy. We all hang around talking about girls at one point or another in our lives, and usually the consensus is to go talk to her if you want to get a date.
Men get nervous and shy like anyone else. Sometimes if you have known a guy to be all macho and all that except around you, it's probably a good sing. Changing behaviors is normal because they want to be on their best behavior to try and impress you. Even though it may seem weird or like the guy is just messing with you, sometimes you just have to ask them out and see if it works because some guys have so much anxiety it will never happen otherwise.
I'm going to leave it at that because the question really depends on age factors, where you live, who you wish to attract, and many other factors. The Internet has a lot of tips that may help on relationships, but in my eyes every situation and person differs, and so general tips can only help to a certain point.
1 - http://mandecoder.com/24-signs-a-guy-likes-you-decoding-his-body-language