Apart from going to the doctor - how can i tell if i'm lactose intolerant?

Trial and error: Eat lactose contaning foods with and without lactaid pills. If no GI upset with the pills, then you might be lactose intolerant.

Self-check: Drink milk--a glass 3-6 times over day. If you get GI upset, lower GI distress (cramps/gas), note this. Then do your best to do without dairy for 3-5 days. If symptoms disappear, is your likely answer--can always "rechallenge" by drinking milk again after a quiet period to see if it brings symptoms right back or not.