A good ad is a marvelous, magical thing. An object of emotion as well as intellect. A work of art.

Once you've done one, you know what a good ad can do. Turn nerves taut. Make fellows mellow. Raise eyebrows or raise hopes. Inspire or intimidate or influence. Impart information that motivates action.

A great copywriter once wrote, "A good ad is like a good sermon: It not only comforts the afflicted, it also inflicts the comfortable."

But the question of the moment is this: How do you know -- before a single living colleague, client or consumer has laid eyes on it -- that you've done a good ad? That it's the right time to stop all the thinking, talking, writing, doodling and designing. The right time to click on "save" and call a meeting?

It ain't easy, knowing that moment. Because a good ad isn't like the 99-yard run kickoff return that everybody in the stadium can follow as it turns into a touchdown. Or the 4th of July fireworks display that gets everyone oohing and aahing in unison.

A good ad is hard to recognize. Often because it's hiding in blah advertising meetings and windy memos. Lost in dim product descriptions and lengthy creative briefs. Or even gone missing inside another ad.

A good ad is difficult to get your hands on. Like a glob of mercury on a glass tabletop. Slippery and elusive. (On the other hand, Leo Burnett said, "I have learned that any fool can write a bad ad, but it takes a real genius to keep his hands off a good one.")

A good ad will sometimes show itself when you least expect it. While you're in the shower, at a movie, listening to the latest from Eminem, or having a couple of quiet beers. Sometimes, even when you're working on something else.

Every now and then, a good ad will sneak up on you from out of the blue. Or from within yourself. A dream, a hunch, a personal experience. David Ogilvy, in The Art of Writing Advertising, wrote: "Some of the good (ads) I have done have really come out of the real experience of my life, and somehow this has come over as true and valid and persuasive."

A good ad is a subject about which you'll hear a lot of views. A lot of people will tell you an ad is good if it wins awards. Some will say a good ad is one that "sells product." Others will say an ad is good only if it "tests well." And more cynical others will say a good ad is "any ad the client buys."

John Caples, who created enough good ads in his career to get him into the Copywriters' Hall of Fame and the Advertising Hall of Fame, opined that "... you're almost sure to have a good ad, if you come up with a good headline." And Bill Bernbach of Volkswagen "Lemon" fame believed that good ads are often the ones that "take chances."

Still, while all the preceding identifiers may be interesting, they are all descriptors after the fact. None of them tells you how to know, at the moment you've done it, that you have done a good ad. How do you decide when to take this beast you've created and lock it up in a cage for all the rest of the world to see?

Two little words: Your gut.

Intangible, unsupportable, unprovable. But unbeatable.

You may be suspicious of it, but you know it's never failed you. You can't evaluate it easily or readily define it but, deep down, you know you can't ignore it.

You know a good ad when you know it in your gut. And that's an easy thing to know.