Examine A Wheelchair
To be able to examine a chair and be able to troubleshoot a problem you should look at the manual that came with the wheelchair.
Out of box, the brakes can sometimes be too tight for the person to use or too loose, whereas the user or caregiver cannot stop the chair with a good response time. Usually you can adjust certain parts like the brakes by turning the brake pressure regulator right above where the brake lever or handle would be.
It is very similar to a bicycle braking mechanism, where you can turn the brake clockwise, or counter clock wise to either tighten or loosen the brake mechanism. The best option as far as examining a chair would be take it to a professional at a wheelchair store or shop. The alternative would be to take it to a bicycle shop, where they provide service for truing wheelchair rims, adjusting brakes, and general maintenance.