In order to avoid being out of compliance with Air Carrier Access Act, UA and other airlines will not question the authenticity or make an interpretation of the legitimacy of someone's disability when they self identify as needing assistance.. Plenty of people can have disabilities that aren't externally apparent. As far as UA is concerned they don't care to know the specific details of what the disability is but rather their focus is on what type of accommodation the passenger needs and if/how UA can provide it.