What is the rule of thumb for assessing whether an M&A deal will be accretive or dilutive?

In an all-stock deal, if the buyer has a higher P/E than the seller, it will be accretive; if the buyer has a lower P/E, it will be dilutive.

On an intuitive level if you’re paying more for earnings than what the market values your own earnings at, you can guess that it will be dilutive; and likewise, if you’re paying less for earnings than what the market values your own earnings at, you can guess that it would be accretive.