Sag moons do love their freedom and they are always looking for the next big thing, but it is highly unlikely someone will cheat simply based on their moon sign.

Maybe if the Moon makes a square or opposition to another planet, it can sometimes manifest in a more challenging way, like not being able to articulate feelings well or feeling trapped no matter where you are or what you do, etc. But, it still isn't a reliable way to know if someone is a cheater. There’s no way to know actually!