Do one or multiple of the following Bungie: 1. Marked targets are highlighted and can be seen through walls. 2. Marked targets take additional damage from all other sources (including if you switch weapons. So basically the fang that marked it doesn't do additional damage) 3. Marked targets put a green glow (like the thorn DoT) over invisible enemies. 4. Target mark is a default upgrade (one listed under the damage type)