The answer is that moissanite will pass the diamond test. Since diamond testers evaluate thermal conductivity, moissanite can be mistaken for a diamond because of how similarly moissanite and diamonds conduct heat.
You can tell a moissanite from a diamond using a particular Moissanite tester that measures electrical conductivity.
However, using a loupe to peer through the top of the jewel at an angle is the simplest way to tell a diamond from a moissanite. The distinctive feature of moissanite stones is a twofold refraction, which appears as two slightly blurred lines.