You need to have your scouting reports research up to par in order to actually make these determinations as accurate as possible. However, if you're looking for this research you'll find it under the Military tree towards the top.
My recommendation is that if you're aiming to kill troops, this video is going to keep you informed on how you can actually determine that after a scout and how much it actually reveals about the potential death ratio your enemy can potentially face even after all his troops go to his infirms!
This is insanely useful if you're wanting to maximize that kill ratio and get your way up to a billion kills! If you like tutorials like these, help support the channel by subscribing!