How do you kill weeds in fake grass?

You can use any regular weed killer such as Roundup: just make sure to use mask when using chemicals such as Roundup, it won't hurt the artificial grass, but such weed killers might be harmful for human, so to avoid inhaling or skin contacting of Roundups while using it to kill weeds. It is very interesting that we find more and more people tends to leave the weeds alone as they sometimes present more overall "realistic lawn" look.

If not much of weed growing, you can always pull them out easily. Normally such weeds grow along the artificial grass lawn's edges or through the drainage holes. Should you know about the weed problem before you install artificial turf, you can always put weed barriers first, then install the turf.