Three tests.
One: repeatability test. Have someone say the slogan back five minutes after hearing it. If they can, it's memorable.
Two: cut test. Can a word come out without the line losing impact? If so, out it goes.
Three: direction test. Does the slogan help you make decisions on later choices (venue, entertainment, catering)? 'Does this fit the slogan?' should become a usable question.
If it doesn't work as a compass, it's decoration. A slogan that passes all three tests is ready. One that fails one needs another round.