I think the pediatrician has to diagnose colic. My son had terrible constipation issues when he was young, came to find out he didnt tolerate the dairy I was eating that was making its way into my breastmilk. Are you breastfeeding? If so you might try cutting out all dairy (milk, cheese, yogart, etc), spicy food and caffeine and see if that helps. You might also try Gripe Water (Mommy's Bliss brands the best), it helped soothe my sons discomfort.
I've never had to deal with colic, thankfully. It's a topic that comes up frequently, so if you want to check out what other moms have had to say about it, you can look here: http://www.mom365.com/Search/CommunitySearchResults.aspx?q=colic