In addition to the background, if you can see earrings, that is usually a giveaway. The generated faces seems to always have different earrings in each ear. Also glasses disappear into the skin, and ears are often different on each side and/or distorted.
It seems the model often doesn't do exactly round pupils either.
I'm 10 for 10 when I use the background to work it out. There are always little imperfections and whirls that give it away.
At least in these images, the skin and clothes are also not great if you zoom in.
Also look at ears and earrings.
If they're wearing glasses do the glasses look they were beaten into shape with a mallet?