There’s no way to accurately detect it. There are various ways to make a guess, and I believe the only reliable way was patched.
One way I’ve heard is to use UserInputService:GetLastInputType(), which returns an enum value indicating the type of input that was last used by the player. For example, if the player is using a keyboard or mouse, the method will return Enum.UserInputType.Keyboard or Enum.UserInputType.MouseButton1. However, if the player is using the UWP version of Roblox, the method will return Enum.UserInputType.Gamepad1, regardless of the actual input device used. This is because the UWP version of Roblox emulates a gamepad input for compatibility reasons. This may have been patched - you’ll have to test it.
Let me know if it works. There’s also another devforum post on the same subject, but no one ever gave an answer.