I hope yours do better than mine. I grew my first one this year and it started off great. They do grow like weeds. Then I think it got spider mites. While I was spraying Garrett juice on it every few days the series of stroms hit and that was it. It was so weakend and stressed that it didn't make it thru the storms. The branches seem so much more fragile than that of tomatoes.
Anyway, the way you'll see that they're ready; if you've noticed the little "balloons that are hanging off the plants (like where you'd normally see reg. tomatoes) the actual tomatillo grows inside that paper cover and will fill it up. The paper becomes like a husk. Then just wait until they get a bit soft and pick them. Of course, don't wait for them to change color.
Just be sure that the tomatillo has grown enough to fill in the outer husk.