Castaway... You are very right, and that method is effective and far less costly than the gauge I mentioned. However, I would add just a bit to what you said if you don't mind...
A paper clip is very malleable (bendable) and makes it harder to "feel" with. Try to use a stiff wire which can be cut to a usable length, but one which, although can be bent, remains mostly straight and springs back when pressure is applied. Bend the last 1/8th inch to 90 degrees, and then sand or grind the tip to a very sharp point. That very sharp point will detect the first hint of narrowing... (NOTE: Tab back up this thread to my eariler reply and look at the picture of the RCBS gauge that I posted... you can see the "feeler" with it's sharp point in it's stowed position...) The end of a paper clip or flat wire is big enough that it does not easily "feel" the developing bad spot...
Remember, the first signs of weakening inside that case is just a tiny, very thin ring around the circumference of the case, not a big anomaly that can easily be felt in your fingertips as you hold and move the wire around... It takes a sensitive instrument, or at the very least a sensitive touch, to "feel" the ring starting to form.
Now again, I'm not trying to impugn your post, just trying to improve on it. I've seen times, using my RCBS CaseMaster gauge, when the gauge was indicating a thinning area, but, in holding and pulling the cartridge case back and forth, I could not feel it as I pulled the needle past the bad spot... I used this "wire" method of watching for "web area thinning" for YEARS before I took the plunge and got my expensive gauge... Probably had one too many night caps that evening... :-D Just my method and IMHO... Dave