Color is primarily how people evaluate the difference however strictly speaking true citrine is due to iron in the crystal matrix giving it a yellow coloration and is rare. Smoky quartz gets its coloration due to interaction with natural radioactive elements and is essentially a burnt crystal. Most citrine that is sold is actually low grade amethyst that has been heat treated.
Definition of Citrine via Mindat.org
Definition of Smoky Quartz via Mindat.org
In essences.. If one has tested to make sure that their stones is a smoky, which is very pale to begin with, can be heat treated to assume the yellowish/orange Citrine color.
Will try to get pictures done this weekend, although color seems to be an issue with my kindle. Seems to lean towards pink! If color isn't good, will try to describe.
I have a couple of citrines from an amethyst area (Kingston Mountain) that were naturally cooked out of the purple! I find about one for every 1-200 xtls.