What does a Tusken Raider look like under the mask?

TL;DR: The image from Anakin may not be 100% trustworthy, yet is probably the best we'll get... and it does fit what we know about Sand People from other sources.

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Although its monstrous quality may be exaggerated somewhat, the face seen in Anakin Skywalker's nightmare (from Star Wars: Republic #62) may be the only existing canon image of a Tusken Raider without its mask on:

The appearance shown here does fit with what little existing information we have on Sand People:

• They are biologically incompatible with humans, and cannot interbreed with us
• They are speculated to share a biological ancestor with Jawas, who supposedly have hairy, spiny faces covered in insects under those hoods (hence the blackness)
• The suns of Tatooine are said to be far more glaring for them than most other species, which is why they wear the lenses and mostly hunt at night. This would certainly make sense if they have no eyelids.
• Tuskens keep their faces covered even amongst each other, so Anakin Skywalker is one of the few living beings to have seen them uncovered.
• Lastly, we know they look nothing like humans. When A'Sharad Hett unmasked himself in front of Anakin, the latter immediately knew that Hett was not a Tusken Raider.

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Apparently, there is some debate online about whether or not Tuskens remain covered up in private. In Legends canon, we have at least two references to this:

• In the "Outlander" comics arc, Sharad Hett told fellow Jedi K'Adi Mundi that he had only seen his wife's face once, on their wedding night. Hett had married a fellow human who was living amongst the tribe.
• In the novel Kenobi, we have the following inner monologue from A'Yark, a Tusken mother thinking about her son:

Sand People lived with sores every day. At birth, every Tusken infant was swaddled tightly in bandages. The nurses worked so fast A’Yark had never seen her children’s faces. Channeled through little mouthpieces, her sons’ cries had been tinny and agonized. Babies had no way of appreciating the curse that existence represented, nor did they appreciate the shame of exposed flesh. But they quickly became acquainted with the price the coverings exacted on the body.