How to check if reset was clicked?

This assumes that you have made all of the scripts in your game

If you didn't kill them, it's probably not unfair to say that they might have killed themselves. When Humanoid.Died fires, check if one of your scripts killed the humanoid (You'll need to set up a broadcast or BoolValue or something to tell scripts that we're looking at murder). If it wasn't one of your scripts, then they reset.

As an example, you can have a suicide watch script like this

game.Players.PlayerAdded:connect(function(p)
    p.CharacterAdded:connect(function(c)
        local hum = c:WaitForChild("Humanoid");
        local murder = Instance.new("BoolValue");
        murder.Name = "Murder";
        murder.Value = false;
        murder.Parent = hum;
        -- local torso = c:WaitForChild("Torso");
        hum.Died:wait();
        if murder.Value then
            -- Killed by a script
        else
            -- Suicide
        end;
    end);
end);

And then in a killing script you can write this

if humanoid then
    -- Ayy let's get 'im
    humanoid.Murder.Value = true;
    humanoid.Parent:BreakJoints();
end

There is one more possibility, which is that the Player jumped off the edge. To catch that, try this:
(Assumes that you've got a Torso predefined from the CharacterAdded with `local torso = c:WaitForChild("Torso"))

else -- If it's not murder
    if torso.Position.Y < workspace.FallenPartsDestroyHeight then
        -- Also not murder, unless they were pushed, but not a reset.
    else
        -- Reset.
    end;
end;