I am by no means the expert here but I've been told to look for her calling a lot and tail wagging. I'm sure someone else could give more signs or more detail.
Being more vocal, mounting other goats, tail wagging, clear discharge, reddened private area.
Of course, some does have silent heats and you'll never know. If this is the case...ask a neighbor with a buck to make you a buck rag. Does in heat will react pretty strongly to them.
My does have all of the classic signs that helmstead listed. I write down the dates so I can keep track of their heat and then take the girls to the buck for a few days to be bred.