Most people cannot tell that a moissanite is not a diamond. Maybe if you had the two side by side, a very keen observer can see that they are slightly different. Maybe you can see that the moissanite has more rainbow sparkles. Or that they have different casts of color under different lighting conditions. But even then, the casual person may not be able to say which one is which. In fact, even professionals have a hard time telling them apart, unless they use a special moissanite tester.