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.
Do you know of any buyers or sellers who have employed the agent? Client referrals are a valid measure of the Realtor's ability and performance. Testimonials provided by the agent are not as reliable as the proverbial "word of mouth" referral. This is as true in real estate as in any other business or profession.
You can then possibly fix the drain yourself with over the counter products. You can then learn how to prevent the drain problem from ever happening again. Another step is to purchase all your own plumbing supplies. If the plumber does not have to leave to get an item that you already have, this will lower the amount of money you will have to pay them. If you do have to call the professional, make sure that you check everything that involves your water quality in your home from dripping faucets to clogs. This way when he arrives, you can have him look and fix everything that you are having a problem with. This will keep from spending the money to call him again.
Ask for written quotes and compare the quality of their services and rates. Lower rates are not always the better choice. Sometimes, those plumbers who offer lower rates provide low quality services and materials, too. Written quotes are also important to give you an idea of what the Trustworthy plumber are going to do and how much are you going to pay for those services.
Now to prevent clogs from happening, you would need a drain cover in order to stop the clogging. However bear in mind that sometimes the best idea is to simply call your Local plumber and have them come out to take care of the clog. It is simply the more financially responsible as well as reasonable. Plumbers have equipment that the standard homeowner doesn't in order to clear the drains quickly and get them flowing again. Just remember that you have to be specific as your plumber is not a mind reader and cannot diagnose on vague information.
Here's an example. Let's say you are a plumber. Will people put "plumber" into Google to try to find you? How about "plumbers"? How about "plumbing"? All of these are valid search terms, but they each reflect a different "conversation" in the mind of the searcher.
Once you have become a master plumber you can start your own plumbing company or you can even work for the government. This may sound like it is going to take forever to become a master plumber and it is difficult to achieve this level but it is not. Yes, it is going to take several years but the prestige title at the end is worth it.