No testing done, I was having problems after eating ice cream & drinking coffee with semi skimmed milk. She said it is common to become temporarily lactose intolerant after chemotherapy and that's what she suspected had happened to me.
She said it was easy enough to tell in most cases as you would know within an hour or so of eating/drinking something with lactose in it. I was having urgent visit to the loo and the runs quite soon after certain foods and drinks so that seemed to fit.
Cheese never affected me though which was handy as I am vegetarian.
Dietician said best way to test if I was still intolerant was to have a little bit of milk and see how I reacted. As I said, I can cope with skimmed but have not tried semi skimmed yet. I am also ok with ice cream now.
As mine was caused by the chemo I don't know if you will be able to go back to lactose mm. For some it is a permanent diet change