Take this example that the site just gave me: https://imgur.com/a/dbpZx2r

It's pretty convincing, until you look in between the lips where the teeth are supposed to show through. You see some very clear "AI smudging" artifacts that are a dead giveaway.

Less obviously: - the eyelashes on his left eye look unrealistic - the way the nose connects to the rest of the face (especially in the highlights) is too smudgy - the texture and tone of the skin is very uneven, and again smudgy in various places, indicating that this is a Mr. Potato Head constructed image.

This 5 year old's shaved moustache: https://freeimage.host/i/26lDkG :)

Following the advice to look for "fakeness" markers I would mention a few recurring ones: 1) unreal background (like monsters or distorted images) 2) strange-looking moles or birthmarks (white, very round or else) 3) distorted texts and logos on clothing 4) fabrics of the garments don't look real too 5) jewelry gets distorted or multiplied 6) glasses often look fake on the edges

Nevertheless this looks impressive.

The pupils are almost always a giveaway. Look closely. Harder to see on darker eyes though.

Teeth, too if they are showing.

The ears often don't match