We had to deal with this issue with a website that hosted 3rd party games, so we were restricted in our options. Since we didn't make the games ourselves thing

We had to deal with this issue with a website that hosted 3rd party games, so we were restricted in our options. Since we didn't make the games ourselves things like server side playback wouldn't work. For us, there was no foolproof method. So our main goal was to ensure that the leaderboard wasn't full of obvious cheating. First, to reduce the volume of cheating, we obfuscated things: 1) We encrypted all communication. 2) We required a single use submit token, generated by the server, with each game play. We also manually reviewed scores. In particular, the top 10 scores of each game. Beyond that, since we required a single use submit token, it means that we knew how long the player took to obtain their score. For most games, the higher your score, the longer you played. So we flagged any scores with an out of whack score/play time ratio for further review before showing the score to anyone else.