What is a cross bite? How can you tell if you have one?

Reversed position. Cross bite means top teeth inside bottom teeth. Can be anterior and/or posterior, dental and/or skeletal. Ask your dentist to check. If you have cross bite, high narrow palatal vault, or jaw shift to the side when you bite down. Ask him to refer you to the cross bite treatment specialist, an orthodontist.
When teeth are shift. When teeth from your lower arch are in front of teeth from your upper arch.
Look in mirror. The lower teeth should rest inside of the curvature of the arch of the upper teeth when the teeth are together. If they rest outside of the curvature of the upper arch, that is considered a crossbite.
Crossbite. A "normal" bite relationship is generally one where your lower teeth fit in or behind (for the front teeth) the upper teeth and they go together like a puzzle. This occurs in approximately 65% of people. Crossbite varies in degrees but is generally when the above situation is reversed for one or more teeth resulting many times in the teeth appearing crooked or out of line with each other.
Bad bite. There are multiple forms of cross bite, check with a professional if you think you have one.