When you request flight following generally you contact the facility (either approach or center) responsible for the airspace you're in, so your real question is "How the heck do I know whose airspace I'm in?!"
The answer can usually be found on the charts, or in other FAA publications.
Let's look at your example (I was over the Pottstown VOR (
PTW) heading to Lancaster (
KLNS) on the 269 radial.) in detail with some convenient visual aids.
Preflight Planning and the A/FD
You don't tell us what airport you took off from, but I'm going to assume
KPTW (Heritage Airport) as a convenient starting point.
If we have a look in the Airport/Facility Directory for
KPTW we'll get an answer very quickly from the "Communications" section:
PHILADELPHIA APP/DEP CON 126.85 (800-354-9884)
So if we're planning a flight out of KPTW and intend to pick up flight following our call to ATC should be made to Philadelphia Approach, on 126.85 (and if we're going IFR and need to talk to them on the ground there's a convenient phone number).
If you have an application like Foreflight or Garmin Pilot this information is readily available to you in the cockpit as well, so if you're unsure who to contact you can always pull up the A/FD information for the nearest airport and use that as a starting point.
In-Flight - Using the Charts
Let's say you didn't remember to look in the A/FD to see who you needed to talk to while you were on the ground, and the battery in your iPad is dead so Foreflight is out of the picture in the air. You can pull out your paper charts and get most of the same information in flight.
If we plot
PTW ... KLNS it puts you on the very bottom of the New York Sectional Chart, and on the Philadelphia Terminal Area Chart for part of the flight, so let's take a closer look at the route on each of those.
First the New York Sectional:
Not much help here - or at least nothing glaringly obvious.
Let's look at the Philadelphia TAC:
AHA! There are helpful white boxes with blue text on this chart that tell you
CTC PHILADELPHIA APP ON and list frequencies, altitudes, and active runways.
This looks promising: Since the little box says to contact Philadelphia Approach that's who you should probably call first.
In your case there happen to be two helpful boxes, one above your route of flight and one below it. Both boxes are roughly equidistant from the route of flight, so it's not obvious which one you should pay attention to. In cases like this you can call on either frequency (at least you're calling the right facility - they may make you call on a different frequency and talk to another controller, but you're in the right neighborhood at least.
You can also go to the Airport/Facility Directory (A/FD) as a tie-breaker (though you're probably less likely to pull the book out mid-air, and if you did you'd probably use the nearest airport like I described above).
The blue boxes are talking about the Philadelphia Class B airspace, so if we look at the entry for the Class B airport (Philadelphia International -
KPHL) we'll get more details on which radio frequencies we should use:
If you're flying along the 269 radial from
PTW you're somewhere "northwest of the airport", so you would call Philadelphia Approach on 124.35 / 128.4 if you're above 5,000 feet, or 126.85 (the same frequency we got from the A/FD entry for
KPTW)if you're below 5,000 feet.
I'd Like To Phone A Friend
There's one final way to figure out who you should talk to, and that's to call someone else and ask. If you're opening your flight plan with Flight Service over the radio they will usually sign off with "For flight following try Philadelphia Approach on 126.85" and the usual plea for PIREPS, and if they don't offer a frequency you can always ask them.