If a particular platform claims to be provably fair, it usually provides a list of ways for you to confirm that claim.
To better understand how this works, let’s take a look at a real example: the True Flip Blockchain lottery. This platform has a quite successful ongoing ICO campaign, which may be in part explained by the principles of fairness and transparency it maintains. Given that, it may be beneficial to take a look at True Flip as an example of best practices.
Right on the main page, you can see the link to the open source code of the lottery, published on Github. By visiting the repository, you can check all the functions which govern the game, including the process of drawing the winning numbers and making prize payouts.
Of course, not everyone is a programming expert, and the owners of the platform should understand that. There are other options to provide proof of fairness. Returning to the True Flip example, the lottery offers a tool which anyone can use to double-check the winning numbers by entering a publicly available Bitcoin hash, over which the owners of the platform have no control.
These two examples don’t cover all the ways that online game organizers can use to prove its fairness. In fact, there are many other aspects of any game, which can be omitted, and consequently used to cheat the players. So the ‘provably fair’ characteristic is more of a scale, where some games may be more transparent than others.
Ultimately, it is up to you, the player, to determine whether a game is transparent enough for you to entrust it with your money. Caveat emptor, as they say, so stay careful, always stay on the lookout for suspicious things, and you should be all right.