Artifacting occurs because the jpg compression algorithm majority of information in an image is stored at low frequencies, meaning that there aren't abrupt edges that are important to the image. This works well for photographs but it doesn't work as well for text or anime due to the importance of the crisp edge.

The best way to find artifacting is to find a hard diagonal or curved edge between 2 flat colors and look for slight blocky discoloration in the pixels around the edge. If you want to see extreme jpg artifacting as an example just go on Facebook and see where somebody uploads an image of text. Facebook's compression algorithms are so aggressive that anyone can see the artifacting.

otaku_emmy said:
And we don't accept PNG with heavy artifacting.
In my opinion, PNGs with any artifacting should be rejected as they're clearly not from the original source. PNG is a lossless encoding and should never have artifacting.