That's a very good question, and for me, I don't feel there is an easy way. Do it slow and painfully, or spill it all and let the chips land where they may.
I know my wife fairly well, and have tested the waters on more than one occasion and it has never turned out well. So I have let fear drive me and keep things a secret against my better judgement. Unfortunately secrets have a way of coming out sooner or later, and secrets also tend to put an even greater strain on a relationship.
Learn from my mistakes, don't wait forever, it just makes things more difficult and prolongs the inevitable. Best of luck in opening up to your partner.