I am following this recipe to make stir fry lamb.
However, I always end up overcooking the lamb out of fear of serving raw food, resulting in tough pieces of meat.
How do I tell when the slices of lamb is done when stir/pan frying?
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.
Cook the meat rare before slicing, in a pan or the oven. Steam the vegetables. Heat oil in the wok and add one plate worth of food (especially if it is a small wok over an electric range). It should only take a short amount of time to fry the already cooked ingredients, and one plate at a time is a nuisance, but it means the wok is sizzling hot for each plate.