Check for natural inclusions under magnification
Look for uneven color distribution
Examine the stone for bubbles, which may indicate glass
Test hardness; ruby should scratch most other materials
Observe fluorescence under UV light
Compare the color to known ruby standards
Inspect facet edges for wear consistent with natural stone
Request a gemological certificate from a reputable lab
Have a jeweler perform refractive index testing
Verify whether the stone is natural, synthetic, or treated