Professional jewelers must undergo years of training and learn how all their specific equipment works to help tell the real gems from the fakes. Rubies hold special renown for their bright red color, and true gem quality red rubies are much rarer than diamonds. Learning how to identify a real ruby from a fake one, or the similar looking red spinels which often times people mistake for the real thing. With proper equipment and training, a person can learn to find the common signs of a fake.
Get a clear light source, like an uncovered light bulb or a lamp without the lamp shade. You will need this to use the Hodgkinson Method, a simple way which helps to identify likely rubies without having to use any special equipment.
Hold the gem extremely close to your eye. You must hold the gem almost as close as a contact lens, as you'll be looking through the gem.
Look at the light through the gemstone. You should see a rainbow reflection in the gem. A full rainbow means it's most likely not a true ruby, as most green and yellow light will be absorbed by a true ruby.
Look for a reflection of red and blue. Rubies normally give off a red and blue reflection, while yellow and green colors in the rainbow mean it is likely a different gem.
Examine the gem from multiple angles, seeing if any angle reveals bright yellow or green light in the spectrum. This could be a sign that the stone is not a ruby, but a similar looking red rock.