How to tell if stir fry lamb is done?
When any meat is sliced thinly you know it is done when it is seared on the outside, that's really it. The 6 minutes in the recipe sounds like too long to me depending on how thinly you've sliced your lamb, I'd halve the time myself. 30 seconds with the lamb, then add the garlic, stir fry for 1 minute and thirty seconds, then add the herbs until wilted, then out. 

That's assuming you've got a really hot burner which can do genuine wok cooking. If all you have is a wimpy medium-size burner then the 6 minutes is realistic. In that case I'd slice the lamb a bit thicker and add a bit of the reserved marinade and a bit of water to keep things moist after frying off the garlic for a minute.