How do I check if the player respawned?

For this you'll need to utilize the PlayerAdded and CharacterAdded event on the server end to achieve this.

Here's an example.

local Players = game:GetService("Players")

Players.PlayerAdded:Connect(function(AddedPlr)
    AddedPlr.CharacterAdded:Connect(function()
    print(AddedPlr.Name, 'has spawned!.')
    end)
end)

Hope this could be any help for you.

Use the .CharcterAdded event.

Player.Character.Humanoid.Died or Player.CharacterAdded