I don’t understand why you want to do this?
A good to detect flying exploits can be raycasting downwards to check if the player is in air for a huge amount of time. Which can be done in the server side. Not the best option, but you can probably get the idea.
But I would request you not to waste time in trying to secure the client, even if you make it really hard, it will still be exploitable as Client has access to all the client side files. Securing your remote events and similar things is the best method for securing the game from exploiters.
Also a really good thread that explains much about common exploits, which might help you get a better understand.