If you drink a lot of milk and get really gassy that's a sign.

Trial and error I guess

Looked it up and apparently there's a test you can take involving drinking a drink with lactose and then taking a blood sample.

Oof

Ok, so I fell down a rabbit hole .. but there are plenty of tests you can order ranging from sending in a simple swab for DNA testing to blood tests.

Drink a gallon of milk. If you survive then you're good.

I think i can rule out lactose intolerant. Turns out I have a light 100 dg fever