That is a good question, is there a way to actually TELL if you are lactose intolerant? I switched to almond milk, as much for the fact that it doesn't turn sour as anything, but I still eat ice cream sometimes when I want some...
I think that some people become lactose intolerant because they stop drinking milk for a while and their bodies aren't used to it... But it you start to drink it again regularly, it will get used to it and not bother you after you get your 'tolerance' back....