Not being one to take SG readings on a regular basis (i.e. every day during primary), I use a 'zen' approach. During the primary, I do check on my yeast buddies every morning and evening and after a few batches, you can get a good feel when low krausen builds to high krausen and then fades to late krausen. Low krausen is the phase when all the O2 is consumed and anerobic activity begins and bubbles start to form on the surface of your wort. The krausen will go from basically a bunch of bubbles to the classic 'rocky' formation that's 1/2 to 1" thick. High krausen is when the ferment is in full swing - the foam on the top will be anywhere from 1-3" (or more) thick and there will be visibile activity in the volume of the ferment. Late krausen will be when the head starts to fall and the visible activity in the volume slows. This is when you want to bump the temperature up to promote any diacetyl et.al. scrubbing. I stick the "outdoor" probe of an indoor/outdoor thermometer on the outside of the carboy and check it twice daily. As the fermentation kicks in, the temperature will rise by several degrees. When the temperature starts to drop, that's another cue to turn my heating pad on. But as Thirstymonk advises, don't cook your beer, just warm it up a few degrees gradually.