Ask around. Ask your friends, ask your neighbors and get referrals. Nothing is more powerful than the testament of someone who has had a local plumber in their home and can speak to his/her capabilities.
If you need an emergency drain repair, toilet repair, or some other plumbing job completed you'd be better off to go online and find a proper business. The best businesses are almost always ranked towards the top because Google is so accurate with their searching relevancy. So if you type in your city and the word plumber you will be given the best in your area!
This one could be a toilet problem. You recall that the toilet tank in the ladies washroom developed a crack last year. Even though it was only a hairline crack over the course of a holiday shut-down you arrived to find quite a large amount of water making its faucet replace way across the tiled floor and out onto the carpet.
Check on the plumber's licensing and insurance. You can ask about this over the phone before you even make an appointment. If the Trustworthy plumber, or his representative on the phone, cannot provide the master plumbing license number quickly and easily, that's your cue to move on.
Call a Local plumber to set up an appointment for an inspection. You will need to be home for this service and when the work is complete, you want to be available to find out what the professional found out. If there is work to be done, depending on the severity, it might be a good idea to go ahead and schedule the next appointment for repairs.
As precautionary measure, you have to tell all your tenants where their individual water shut off valve is located, as well as the entire building's so they can turn it off in case of emergency. And don't forget to make them accessible. It won't do any good if they know where it is but it's locked and they can't break in.
Without a knowledge of this home improvement area, you run the risk of creating more of a problem than you began with. You don't want to make things worse and you don't want to create any more damage, so take a step back and call a knowledgeable plumbing service to come in and take over the project.