On my installation of Inv 2009 (and was this way for all prior versions back to R10, default color settings) the sketch lines change from green (unconstrained) to black (fully constrained).
In 2009
#1 line colors change as stated above.
#2 in the bottom right hand corner (around the memory meter area) it will also tell you how many dimensions you need as well as saying when its fully constrained.