I just found an old jar of Beef Bouillon cubes and although there is no expiration date, I'm a bit skeptical about using them - do they go bad? 
Taste them. Bouillon cubes contain enough salt to preserve them from spoilage, but the flavor (which, after all, is why you're using them) may weaken, dull, and change over the years. If you still like their flavor, then go ahead and use them. 