Keep a pack of gum on you. Take a piece then offer your potential partner a piece. When you become comfortable in the relationship you should say something about it in the kindest way possible. More than likely, they already know about the problem and are uncomfortable with it.
You will just have to tell them. Offer them a mint and if they refuse, just say “no, really you do need one.” Say it with a smile on your face. If it gets embarrassing you could go with something like “hon, you are totally hot, but not so much your breath.”
I like the idea of offering gum. Also, depending on your level of intimacy with this person, you could suggest brushing your teeth together before bed…or carry a packet of those dry use toothbrushes and offer to share that with him instead of a piece of gum.
Hand them a toothbrush and toothpaste…problem solved. Seriously just say “whoa” you need to freshen up and they will gladly fix the problem if they have any interest in locking lips with you.
Bring up the topic in general giving an example of a co-worker/customer/friend with bad breath who insists on talking within smelling distance. Man how nasty it is! The conversation may flow to a point where the person asks if they have bad breath, or they may take note of your concern and take preventive action on their own. Unless they are oblivious, they will take the hint.