While it doesn't warn you, per se, if you have the little activity display enabled it will show friendlies in green and enemies in red. You can click a red name and if they're in range it will target them. I just keep it small and at the bottom right of my screen. If you have the minimap icon enabled, you can either right click or left click (don't remember for sure) it while you have an enemy targeted and it will prompt for a comment and then add the name.