I think a key part of eating the right amount (especially if you’re coming off a long stretch of eating more than you ought) is asking yourself the above question with some regularity (i.e. several times during a meal).
YMMV, but for me, the “I’m not hungry any more” signal is what we geeks call “edge detect”. It comes once, and it’s soft and easy to miss. If I miss it, or ignore it, the next signal is “man, I’m full”, and the one after that is “man, I am stuffed.” (That one tends to stick around!)
But if I ask myself “am I still hungry?”, and stop eating if I’m not (this is actually the important part! :), then I can recover that lost or missed signal.