I'm not really sure I've found it. God knows I've tried.
I've probed every inch I could possibly reach... with objects large and small... in every position I could contort us into. What I've found is that it depends on the chick. Some are into any kind of probing, and some aren't into any kind of probing.
Some chicks are only into it because they know it's what we want. But they're still into getting you off. I guess that's what counts.
Focusing on the clit is always a safe bet.