There is no law that requires a company to search and clear a trademark before adopting it for use.  However, failure to do so can lead to all sorts of trouble.

Searching and clearing a trademark prior to using it means conducting research to see who else might be using your trademark before you.  If you don't clear a trademark before using it, not only might your mark be infringing an existing mark, but you may be deemed a "willful infringer" if a court decides you purposely turned a blind eye to avoid finding out that you might be infringing.  

Generally, the best practice is to do some searching & clearance work before adopting a mark, even if you just do some of that on your own using the DIY guides on this site or elsewhere.  Beyond that, some companies spend thousands on expensive search reports and legal opinions analyzing those reports. This money is well spent in most instances, if you can afford it, as most small mom-pop shops generally cannot.  Those who chose this path are rewarded with avoiding trouble before it begins and a greater peace of mind that your trademark really is your own.  

Overview of the Search & Clear Process

Start with a knock-off search by typing the exact trademark word(s) in a couple different search engines and click in several pages deep to see if you spot anything.  Also go to the USPTO.GOV website to search all the registerered trademarks, and those attempting registration as well as those previously registered before expiring or being canceled.  It's important to get well versed in the legal rules that dictate what constitutes an infringement risk or not.  It's not what meets the eye.  

For this reason, the second part of the search & clearance process, namely the clearance part, usually involves getting a legal opinion from a trained trademark attorney.  Their opinion will often alleviate concerns considerably, thus clearing the way for you to begin using the trademark and even registering it you wish.  Sometimes even a second legal opinion may be useful in close-call situations.