To make things clear, When the player plays against the house, like in slot machines, the game is considered a chance game. When a player competes against other players, though, it is considered a skill game. A player can tell if a game is skill-based if it combines statistics or math skills or techniques.
This is not really accurate, regardless of who you are playing against as the name implies a skill-based game is one where you skill has an effect on the outcome of the game itself regardless of how much luck its inherent to the game, for example poker and blackjack are two examples of skill-based games in which your skill has a great effect on whether or not you can win but the main difference is that in the case of poker you are playing against other people while in the case of blackjack you are actually playing against the house.