If you arrived on an international flight, then it's very unlikely that you exited the airport without passing through customs.
It's most likely that you just didn't recognize that you were were passing through customs, because I'm guessing you were expecting some sort of search of your goods or at least a desk where you'd have to explain what you were carrying, etc.
However in many airports you have the option of walking out through a "green" nothing-to-declare zone/passageway which is more-or-less the equivalent of you saying that you don't believe that you're carrying anything of interest to customs.
Often in my experience these green zones don't even have anyone on duty in them, but sometimes they do, and sometimes a passenger passing through the green zone will be stopped and have their belongings checked to make sure they're not trying to import anything they shouldn't be.
So as long as you left the airport the same way all the other passengers did, and didn't sneak out through a side door or something like that, you're fine.