We can take it for granted that all elephants being used for tourist rides are and have been brutally abused. There is no other way to make a wild animal conducive to being ridden. The elephant when used commercially is like a piece of equipment that earns money hand over fist.
You can look for tell-tale signs of physical and mental stress including cracked toenails, overgrown, cracked footpads, visible wounds and scars, blindness (opacity of the corneas), old age or dehydration (sunken temples), malnourishment (prominently visible spines and hollows in the head), and stereotypy (repetitive, monotonous motion like head bobbing, swaying or weaving that indicate extreme mental stress and trauma). You can also check if the elephant is chained, has rope burns and wounds from restraints; or if the keeper is using weapons like spears or a nail embedded in a harmless looking piece of wood to inflict pain on the animal, or shouts loudly in order to make their elephant obey commands.