DON'T. You said you are best friends, and if he thinks that's what the relationship is, telling him you have let it go to a place he hasn't, could ruin the friendship. You need to ask him to tell you about this person he likes. IF IT IS NOT YOU, you have to decide if you can still leave it as friends only, or let him go completely. IT WON'T WORK BOTH WAYS. THIS is why it's so hard for men and women to just be friends.
Don't say anything at all. If you are the one he's into he'll tell you when he's ready. Bringing it up beforehand, especially when you are not sure yourself, would just be adding more awkwardness to an already strange situation and could end up driving him away, destroying any friendship you already have