I invested in a weighing scale - the digital ones for the kitchen that goes up to 10lbs. I bought mine for 12 bucks at Target.
I weigh my snakes probably once a month or so. When they stop eating, I watch their weight. If they lose more than 10% of their weight, then I would take them to the vet. I've had some of mine go off feed for a few weeks - almost 2 months once. But, their weight never dropped, so I didn't have to take any of them to the vet for skipping meals. The scale is pretty much a "comfort" thing for me now. I see no change in weight and I feel better about it. Then I can be happy for the rat (the breeders are my pets) that got to live another week! Yep!