By definition, the douga is what was directly 
used to make the cel. It is where the lines on 
the cels were copied from.

A genga is a sketch done by the hop artists that 
is done in order to set the mood and positioning 
of the scene. It may be rough and quick or it may 
in some cases be done even better than the actual 
dougas for the sequence (lodoss OVAs for 
example). It all depends on which animator is in 
charge of a scene. 

Often genga may be on colored paper, but a lot of 
rough sketches (things done to practice or get 
the drawing right for the douga) are also done on 
colored paper, so that is not a good rule of 
thumb to use.

The one thing that genga have are usually the 
sequenced number circled and drawn in the main 
field of the cel (i.e. not the corner, but right 
in the center of the cel). This means the sketch 
could not have been used for the douga and it was 
of a key animated sequence. If you see this, you 
pretty much know it has to be a genga.

For more info follow these links

Yann's and Brian's sites

Catch you later,
Marcus