If not cheating is more important than the game itself, try to construct and present your game in a way that it looks like finding the solution to a math problem. So the server will give an instance of the problem to the client (example A: a chess board about to be won in 3 moves, example B: a geometry dash randomly generated level) and the user will have to solve it and post back a solution (example A: the winning moves, example b: the exact timestamps and intensity of jumps to avoid obstacles)
With this approach, it is key that the server doesn't send the same level twice, or else the cheater can plan and "design" his solution in advance. Also, the game information must be randomly generated in the server and not sent via seed, or else the cheater can fake the seed and design his solution with time.
The given time for valid submissions must be also tracked in the server so that they will only have "playing" time and no "designing" time. If the cheater is good enough to design a solution as fast as honest players can win the game, then they are talented enough to win the game honestly and deserve their points.
Back in the server, you will need to check that the submitted solution is valid for that instance.
Of course this approach requires lots of extra work: More instances of games (ideally infinite and non repeating), server side generation, server side validation of submissions, time caps, etc.
Note: I know these approach was already suggested in multiple solutions some years ago, I wanted to add my humble contribution.
