How do you know if you have become lactose intolerant?

Symptoms after dairy. Can occur as we get older, more in some ethnic groups (jews, blacks, asians), with certain parasites (giardia), after gastroenteritis. "poor man's test": drink 1 pint skim milk on empty stomach; get symptoms (gas, bloat, cramps diarrhea within 2 h). Can do lactose tolerance test (blood or breath test). Can have small amts as mixed meal; fat helps tolerance; yogurt more tolerable.
Lactose intol. This is a subjective diagnosis. If the diarrheas or abdominal discomfort produced by dairy products improve avoiding them you have the diagnosis. Pts may try lactaid instead with improvement. There is no good test for it.