aaronw wrote:
So let's say I'm flying out somewhere (VFR) and I decide I want VFR
FF. Most of the C-172Ss I have flown in have the NRST on the GPS. So
I push NRST and I choose 'CTR'. So this gives me the center
frequency. All good and fine, but what if I am close to the radar
(TRACON) service area of a Class C and actually the airspace I am in
is the domain of that TRACON? I assume that the TRACON area of
responsibility is a little further out than the B/C rings

Yes. The airspace owned by a TRACON typically extends at least 20 miles
out beyond the edge of the charted airspace. Sometimes quite a bit
more. As far as altitude, they often control well above the charted
airspace ceiling -- 10,000 AGL or even higher is not uncommon.

I am assuming the 'worst that could happen' is that I call Center and
they say 'no, dummy, you want to call XXX approach on YYY'.

Exactly, but they won't even call you dummy. They'll just say something
like, "You're outside my airspace, contact XXX Approach on 123.45".
Happens all the time.

My question is - is there any way to find out this information
beforehand?

The best reference is the AFD. Look up the entry for the airport
nearest your location, and see frequencies it lists for Approach and/or
Departure. To be honest, I don't usually bother en-route. I just call
up the nearest TRACON and if it turns out I've guessed wrong, they'll
give me a different frequency.