The most frequently used symbol for disability is a wheelchair, which tends to reinforce the common preconception that wheelchair users are the only 'real' disabled people. However, most disabled people do not use a wheelchair and very few use wheelchairs on a full-time basis. In addition, for a variety of reasons, many disabled people do not chose to call themselves disabled.
Availing of access concessions etc can be problematic for people who have conditions that fluctuate or are hidden from view.
Hidden disability affects people in so many different ways that the best advice is to use your imagination and, rather than 'policing' people for use of services, listen to what they have to say about their access requirements.
For example, do you really need to know why the non-wheelchair user has asked if you have an accessible washroom or a lift. Or why someone is insisting they want an aisle seat near an exit rather than being 'stuck’ in the middle of a row. The person injecting themselves is more likely to be diabetic than a drug user and that wobble in the walk or the slurred speech may be more to do with cerebral palsy, epilepsy or the remnants of a stroke than with alcohol abuse.