Agreeing with Franklin. Taking the specific gravity readings over 2-3 days is the best practice. You could also pick up a refractometer to take gravity readings if you need to. I use one since I do 1-2 gallon batches frequently and only larger batches when I know the recipe is good. EDIT: As a note from a commenter, refractometers do not give direct readings in the presence of alcohol; you'll need at least a correction.
However, just because there isn't any more alcohol being produced or visible activity doesn't mean 'fermentation' is completely done. Fermentation isn't just about making alcohol. I assume you've heard of some of the byproducts that yeast make while eating, pooping, and reproducing. Some of them are not good BUT yeast love to take care of themselves so after the alcohol is made they will also clean up some of those nasty byproducts, cleaning up the flavor of the beer.
My novice suggestion would be to raise the ambient temperature of the fermentation chamber or just wrap it in a sweater or blanket. Three days is usually when the frequent activity stops for a lot of my brews as well but I always let them sit at the least a week. It can't hurt!