They actually are trained to go on command. You have a schedule where you know they have to go every few hours. You take the harness off, tell them to do their business, put them back on the harness and off you go again.

You will usually develop a relieving schedule with your dog, so that you both know when potty time is. When you keep those times on a regular schedule, you don't have to worry about your dog needing to relieve themselves while walking around on the job. Sometimes you'll just have an extra opportunity here or there, or you'll know you'll be out for a while, and you will find an appropriate place and tell your dog to do their business. They are pretty amazing creatures, but like all living things, there will be accidents occasionally. People are usually much more forgiving of a guide dog's oopsie-daisy than a person's though, so dont worry!