You have no in-game way of checking if you have made any kills, so be very diligent. Frustrating caveats that might cost you this achievement if you're not careful.
(1) Environmental damage. If someone dies to falling, electricity, gas, etc, it's attributed to you.
(2) Turret/Robot domination. Turrets that kill hostiles are also attributed to you.
(3) Due to some bizarreness with the game's physics engine, I found knocking out someone with either the Stun Gun or the Tranquilizer Rifle would sometimes have the enemy fall and ragdoll in a way that would kill them. I don't know precisely what causes this, but I had it happen very consistently with specific areas of the game world. Check anybody you neutralize this way to make sure they have no skull icon. Strangely, I never got someone killed from a takedown this way.
(4) Although the bosses themselves do not count, killing the insane people during the final boss fight absolutely DOES count.