I listen for the flush water fill to change it's tone, you can usually hear the water hitting the sides of the tank that will stop when its almost full, then it's time to pull the plug. I genearlly will do 3 or 4 full or 3/4 full pulls and using the clear plastic conector at the hose conector helps to see the condition of the flush.
Updated edit.
I also have the Macerator pump if I need to do a long run such as when I come back from Q-site, the only down side of the pump is that it is a longer process to pump the Poo than to use a 3 inch gravity dump. I have attached the web site of the pump that I have but there are other pump manufactures out there that do the same thing.
http://www.emptythetanks.com/index.html