That was fun. I started out really well - in the 80's but dang I can't tell the difference between the APS and 4/3rds that much especially with some of the images that have been altered or are blurry.
What I noticed was the few full frame shots I missed and guess they were APS-c … they were all from Nikon full frame.
And the few I thought were full frame but weren't were all the Olympus OM-D
I have been thinking about giving m43 a new try.
I looked into it a few years ago with the Panasonic GF1 and GH2 but I was not entirely convinced.
I have the impression with the E-M5 and E-M1 the image quality has significantly improved.
How could anyone tell from tiny, compressed pictures if they were from a four thirds or an APS-C?
Not entirely IMO. I invested in Medium Format Digital gear about 4 years ago.
I am pretty sure that at that time the differences between the formats was still more noticeable, as was the case for film.
Nowadays these differences are getting smaller and smaller every day is my impression with the best of each format offering image quality that is sufficient for most professional uses.