Rubies and garnets come in a range of red tones and can be difficult to tell apart, though they are very different types of stones---rubies are the red variety of the mineral corundum, which, in its many other colour variations, are known as sapphires; garnets come in every colour except blue and are comprised of a group of six types of similar minerals: pyrope, almandine, spessartine, grossular, andradite and uvarovite. While there are a few key differences that can help you figure out whether you have a ruby or garnet, the only way to be absolutely sure is with the help of a qualified jeweller or appraiser.
Observe the colour. Garnets tend towards a dark red, while rubies are usually a clearer, vivid red--even tending toward purplish red, which is more valuable. They can also be pink or orange-red, but are then classified as sapphires instead.
Note the price of your gem, if that is available. Red garnets (almandines) are easy to find in both larger sizes and small, so they are generally affordable at a few dollars per carat. Rubies are much rarer, with particularly large ones valued higher than an emerald or diamond of similar weight.
Look for inclusions, or fragments of other minerals, in your gem---garnets typically have a lot of them.
Determine hardness. Rubies (and all corundums) rate 9 on the Moh's Hardness Scale, while garnets rate anywhere from 7.25 to 7---both can scratch glass, however, so it's best to consult a jeweller or appraiser for the final word on this quality.
Find a local jeweller or an appraiser. These professionals have tools that can definitively determine the type and value of your gem. The National Association of Jewelry Appraisers lists independent appraisers by city and state.