Like the first image, one is very organic while the other is very geometric. Knowing that one was PC generated it seemed obvious which it was.
Same with the second image, one looks organic, the other seems to have been Photoshoped with blur filters and such, and the sunglasses on the kid are way out of place.
Look at the portrait of the girl in the third set, and look at the portrait in the sixth set. When you are painting with a brush you don't get blur, you get blend. There's also some odd transparencies (red orbs) and such in the third set portrait that stand out. Again, I thought it was obviously fake compared to the boats.
If you've never tried to paint and/or never really looked at actual hand paintings it might be hard to tell the difference. Get to know the basics, and get a good look at a few real paintings and you'll begin to recognize the work. 
There's several paintings you can compare in the list "real vs fake". Like the colorful lamp scene in the second set vs the one in the fourth set. Or the boats in the third and ninth set. Think about how you would paint with physical brushes and tools, and how colored paints mix in the real world. If you mix all the colors of paint you get black, if you mix all the colors on a PC screen you get white.