Hey David, it's pretty easy to tell when sap goes bad. Basically it is sugar water, so as soon as it's out of there tree it starts to break down from exposure to bacteria, yeasts, etc from the air and your containers. If it's cold out this action is greatly delayed. The warmer it gets the faster your sap spoils. Sap that smells a little off or is cloudy is definitely on the way to being bad, but it's not necessarily bad. Often it cooks down into great syrup. But once it smells like bread dough and starts to have a texture to it, it's probably too far gone.
Usually you get 24 hours at least to get it heated up and pasteurized, often much more at the beginning of the season, and sometimes at the end of the season the sap is bad right off the tree. I hope that helps.