Interesting question- slot machines are actually based on a degree of randomness in each individual play, but the payouts when those random events produce a winning combination are controlled to ensure that only a certain percentage of money played is paid back. Typically in the high 90% range.

If you play the same slot for long enough, you should be able to see how often the game pays out and what kind of winnings you get. Using a casino bonus to discover this is a good idea. If successes are rare but significant, it's a high-volatility game. If you win often, but the wins are small, it's low-volatility.