You might be surprised to learn that you can tell if waffle batter is bad simply by looking at it. If the batter is lumpy or has changed color, it is probably time to throw it out. The same goes for if the batter smells sour or off.
Of course, the best way to tell if waffle batter is bad is to cook a test waffle. If the waffle comes out undercooked or has an odd flavor, then the batter is probably bad and you should start over with fresh ingredients.