Big hand gestures is something to look for when chatting with a guy.
If this certain someone is interested in you, he'll use his hands a lot more than usual. Guys tend to use their hands in conversation a lot more when they are talking to a girl they really like.
The reason they do this is to make sure you are understanding what they are saying and that you are on the same level. It is also because they are trying very hard to hold your attention.
That being said, some people tend to speak more with their hands than others, so don't take this as a sure sign he's into you :)