How can I determine a true diamond? Can it be determined by its brightness with hit by a ray of light?

You need a diamond tester. Beware of moisanite. It tests positive with a diamond tester. Get a moisanite tester also.

How can I determine a true diamond? Can it be determined by its brightness with hit by a ray of light?
No. Diamonds, like all other gems, have characteristics like their hardness, luster, specific gravity (basically density) and refractive index. The old test of using a diamond to "cut a diamond" - the theory being that if a known diamond can cut or scratch the surface of an unknown stone, the unknown stone can't be a diamond - only resulted in a bunch of stones with scratches on them and angry people who thought they might have had a diamond.

The easiest way to tell is with a diamond tester, most often using thermal conductivity - which is in addition to the characteristics I first mentioned. The thermal conductivity tester CAN be fooled by such things as man-made real diamonds, treated diamonds (glass filling, lasers used to burn out inclusions, etc.) and moissanite.

The only sure way is to take it to a reputable gemologist who has the experience, equipment and training to tell the difference. As The WB correctly pointed out, a gemologist (or sometimes a knowledgeable jeweler) uses a combination of tests to rule out simulants. It took me years of study and practice to become a certified gemologist.

Hope this helps.

Diamond may often be mistaken for Cubic Zirconia (CZ).

True Diamonds always reflect light whereas CZ reflect different colors.

Its difficult to differentiate the two for a naked eye.