How do I know if a player has reset and not died from another cause?

If you look at SetCore

You are able to see "ResetButtonCallback"

You can assign it either a bool (false to disable reset) or a bindableEvent callback to execute whenever the player requests it.

Here's an example of how to use it.

local reset = Instance.new("BindableEvent")
reset.Event:connect(function()
    print("Reset Requested")
end)

game:GetService("StarterGui"):SetCore("ResetButtonCallback", reset)

everytime the player tries to reset, it will print out 'Reset Requested'