As with any altered image, a quick visual inspection is all that is needed to discredit photos of extraterrestrials. The viewer should be mindful of inconsistent quality, curved and bending surfaces, and unusual or shifting light patterns, all of which are evidence of manipulation.
The most useful tool an audience has for detecting fake extraterrestrial photos is common sense. As recently as 2009, an image of a dead, bloated sloth was mistaken for that of an alien. A more well known example of an image with an unusual, seemingly alien subject that was discredited is the "Face on Mars".
Originally spotted in 1976, following the return of the Viking I mission from Mar's surface, the picture seemed to depict a pile of rocks that had been deliberately shaped into a human-like face. Pop culture latched onto the image, which has appeared in multiple movies and novels as proof of an alien settlement on the Red Planet.
In 1998, following updated images with newer technology, the infamous "Face on Mars" was proven to be an optical illusion caused by the displacement of light on the rocks. Other objects commonly mistaken as otherworldly include military aircraft, lens flares, clouds, planets, and the moon.