Your first step to make sure a plant really is an onion or garlic is looking for the classic shapes of a bulbous root and a rounded stem that onions and garlic share. Once it passes that test, go to the scratch and sniff phase of testing. Scratch the bulb, or bruise the green tops, and you should immediately smell the familiar oniony odor. If you start to have a few tears welling up in your eyes, all the better. Then you know you have an onion or garlic genus member for sure. Unless you're just sad, your crying spell was caused by the same process that goes on in your kitchen when chopping onions. The plant contains numerous sulfur compounds, which mix with the salt in your tears, to create a weak sulfuric acid--the cause of the burning eyes and crying while dismembering these plants.