There would be no way to guarantee, not 99%, that you are okay with a proposed trade name. A service would cost over $100 (US), and since a TM attorney would be best, you're talking hundreds or thousands. You can pay online services to search for yourself, using their databases of TM information. Your searching should include "sound-alike", i.e. if you want to buy Kokakola.com and you never heard of Coca-cola. But even such a search may not flag a Kokacool.com or colakooka.com compared against CocaCola. There are thousands and thousands of registered, active US trademarks that no one has heard of.
USPTO is one source, so is WIPO (World Intellectual Property Organization). If you see a TM exists in, say, Germany, and you are in USA, then you should be prepared to pay to file in USPTO to register in at least one class (there are over 40 classes), in order to protect yourself.
But you could conflict with 2 or more TM's... e.g. CocaPepsi.com. This could also be hard to search for, if your name is 2 or 3 words put together.
And if you conflict with a company's trade name, but they have not filed a TM, they still have rights. As would you, once you put your name online.
There is probably 0% guarantee before you take any action, because you have no protection at that time, and because it is all a matter of perception. A lawyer could file against you - or write you a cease-and-desist letter - if you have widgetyworld.com and their client has a trademark for "Vidget & World".
A lot depends on how each side views their goals, resources, and strategy. If you are building a business brand for yourself, then you will find there are no easy answers, but plenty of practical steps to take with regard to your trade name.
