The "Free From Falls" DVD mentioned in another thread comes with a booklet, "Minimizing Your Risk of Falls." On page 17 there is this (excerpted):
How Do I Know When I Need a Mobility Device?
You may need a mobility device (or a change in your current mobility device) if:
~you are experiencing frequent falls
~you hold on to furniture or walls while walking
~you are expending too much energy when walking
~you avoid certain activities because of mobility difficulties or fear of falling
~you have difficulty maintaining your balance when standing still or moving
~you become fatigued when standing for more than 30 minutes
Mobility devices are defined as orthotics (braces), canes, crutches, walkers, scooters, and wheelchairs.