How do you know you’ve had a good negotiation—you’ve gotten the best deal possible without obliterating the relationship? In the real world, outside the confines of a negotiation class with everyone’s agreement posted for everyone else to see, the truth is: you won’t. You’ll never really know how well you did versus however well you could’ve done. Sure, if you happened to slam-dunk it or bankrupt your company, you’ll probably have a sense. But in most negotiations, whose outcomes lie somewhere in the mushy middle, you’ll always walk away wondering.

So should we all utterly abandon the effort to assess our own negotiations post hoc? Before we go quite so far, let me suggest a simple heuristic that can still offer some clues to your success, thereby making the post-negotiation process negotiable.

The heuristic, surprisingly, is this: Fatigued, not frustrated.

What in the world could I mean? Fundamentally, a “good negotiation” entails sticking to your aspirations, pushing for your interests, and creatively attacking a seemingly intractable set of positions. That’s tiring! If you’ve really done all that, you’ll probably feel quite fatigued—and you should.

But wait, does that mean that the best negotiations are the most unpleasant ones—that we should experience our most successful deal-making as a flurry of frustration? No! Fatigue is far from a synonym for frustration—we can all walk away from social situations feeling sleepy but willing to sleep it off and send a thank-you note. Instead, frustration is probably a sign somebody obliterated the relationship.

But wait #2, why shouldn’t we walk away from our best negotiations feeling happy, wanting to high-five our counterpart and buy them a beer? Because if you feel that way, chances are you folded too quickly and easily relative to your aspirations or interests—or didn’t define them well in the first place.

But wait #3, does all fatigue = a good negotiation and all happiness = a bad negotiation? Of course not. You might feel fatigued because your neighbor’s dog was howling all night or, more germanely, because you just got schooled by a counterpart who totally outsmarted and exhausted you at the same time. And you might feel happy because your neighbor’s dog finally shut up at 10 pm or, more germanely, because you somehow found a magical counterpart who was shockingly amenable to your wildest dreams.

So I’m not suggesting a 1:1 relationship between fatigue and successful negotiation. I’m simply suggesting a heuristic than can help you play Sherlock Holmes on your own post hoc feelings and reactions. So the next time you walk away from a negotiation feeling fatigued, relish the feeling! Or at least entertain the possibility that you performed quite well. But don’t confuse frustration for fatigue and somehow elevate relationship obliteration to a virtue. And don’t assume that overwhelming feelings of joy necessarily flow from the very best deals. They don’t.

Three cheers for fatigue!