How do I find if a Player killed another Player?

In most ROBLOX-created gear, there is a tag (ObjectValue) inserted in the humanoid of the local player that links to whoever kills them. This tag is referred to as the CreatorValue, and tracks the player's Knockout statistics both in-game and on their website profile.

For example, if I were to swing a bat at your character, an ObjectValue by the name of CreatorValue woud be instanced in your Humanoid with its value set to my player. If my assault resulted in your death, I would be awarded a Knockout.

Creating the Creator Value

local debris = game:GetService('Debris')

object.Touched:connect(function (hit)
    if pcall(function() return hit.Parent.Humanoid end) then
        -- line above 
        local tag = Instance.new('ObjectValue', hit.Parent.Humanoid)
        tag.Value = game.Players.LocalPlayer
        tag.Name = 'CreatorValue'

        debris:AddItem(tag, 5) -- give it enough time (5) before removing it
        -- Could also take care of damages here
    end
end

N.B: Not every tool is obliged to follow this tradition, but it would definitely be necessary if you wanted those kills to be reported back to the website (and for updating leaderboard statistics).

Finding the Creator Value

In the script you want to handle leaderstats, you will want to check for the CreatorValue's Value property when the local player dies (Died event fires).

Note: Only the most recent CreatorValue (first value found), will be taking into consideration when scouting through the Humanoid.

local players = game:GetService('Players')

players.PlayerAdded:connect(function (player)
    player.CharacterAdded:connect(function (character)
        local humanoid = character:WaitForChild('Humanoid')
        -- waits for humanoid after character and player are
        -- added

        humanoid.Died:connect(function ()
            --Humanoid dies
            local list = humanoid:GetChildren()
            -- list of all children of humanoid
            for i = 1, #list do
                --iterate
                if list[i].Name == 'CreatorValue' then
                    -- if value at iterator is CreatorValue
                    local suspect = list[i].Value
                    -- suspect is murderer
                    -- do something
                end
            end
        end)
    end)
end)