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 your service provider has adequate knowledge and training on the topic, this might be a good time to find out more about energy alternatives. He may be able to install low-flush toilets and lower-output shower heads. This is also a good time to talk about solar thermal water heating, though perhaps before the roofer is finished up top.
Second, do you have the knowledge required to solve leaking pipes/faucets and clogged drain problems? You should be able to understand the parts of the faucet, pipes, and drain. Various types of faucets are sometimes designed to function very differently. You should know how to dissemble it replace the busted part and water leak put it back together again. If the problem is in the pipes, you should know which one you have to remove and replace.
Leaks can come from so many sources, so you know that this is going to take time to locate and rectify. At least in this case there Trustworthy plumber is actual evidence that can be tracked. A hidden leak can drip on and on for a long time, silently adding money to your water bill.
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.
Water can do more that just get everything wet. It can destroy walls, foundations, structure, flooring, and many other aspects of your home. Minimizing the damage by knowing where the water valves are located, keeping more water from harming more of your home, and having a plumber you can trust to fix your plumbing problem right and for a reasonable is key. We all have to deal with plumbing problems. Be prepared.