Not to split hairs, but the 140A had the factory metal wing ('49 & on). The original 140 did not; but you’re 100% correct that the flaps don’t make any difference except for a tiny amount of extra drag; I rarely use them and find it more rewarding (and easier) to slip it if required instead. I agree completely with you on finding one as light as possible, since on a hot day, the 140 is a pretty slow climber with two adults. I’d stick to fabric wings too.
The way to tell... Blah blah blah warning: rabbit trail ahead...
The way to tell is that the 140 has a jury strut, with two struts going down to the fuselage; the 140A only had one wing strut, like a 150. However, a lot of 140s have been modified with metal wings, most of which are not true 140A wings, but an STC that simply replaces the fabric with metal skins and leaves a bit to be desired in terms of quality, in my humble, non A&P opinion.
The 120s originally had no flaps and no electrical systems, and the round rear windows were an option or added later. The 140s had flaps, electrical systems, and all had the rear windows. Then in the decades since, many 120s have had the flaps added, electrical systems installed, and the rear windows.
Stalling the 140s with the flaps down can occasionally result in a neat (and abrupt) up and over spin entry if uncoordinated. The 140 spins okay (not as well as a 150 though) but the problem is its quite easy to overspeed the flaps in the recovery, so probably not a good idea! They do slow down wonderfully though, and albeit not as good a STOL airplane as a cub or champ, they cruise much faster and do a respectable job on grass and short fields.
Haha, it’s a lot easier than the Champs with the air oil shocks where every landing is really two landings; you touch down the mains and then fly the strut down until it settles, LOL! Then you can dance on the rudders as the tail settles, and then have fun with the heel brakes.

I admit I’ve only flown Supercubs, Champ, various RV’s and 140s that were TW; so don’t have a great breadth of experience, but a bit of TW time in those types. Of that small sample size, I thought the 140 was the easiest to learn to land and ground handle. Not at all to be argumentative, I’d be really interested to hear what you would say would be easier? Maybe the 65hp Cub, or something else?
Me too! I hated having to fly a 172, it’s a kite! 