Sdsinc is absolutely right about commercial value.
If you owned a one-word .COM with commercial value you wouldn't have to ask if it was valuable - because you'd be getting offers, and traffic. And you probably would have paid thousands of dollars for it.
There IS such a thing as too generic sometimes, in that a second word that helps define the industry can be helpful. For instance, compare "Cleaning" with CarpetCleaning, HomeCleaning, Landscaping, or AutoDetailing. Those are a bit more specific, although still considered premium category killers.
If it's a dictionary word, ask yourself whether there are any products associated with it. Type the word (not the domain) into your search engine. Are there advertisements at the top and to the side in bold indicating that companies have paid to advertise that exact product/service? If so, that's a good sign. Then check to see if similar or compound forms of the name are being used, and if it's registered in other extensions. If the domain is aged (i.e., not a fresh registration, but something where the WhoIS dates back several years), even better. All these things are proof of its potential value.
I'd always check Namebio, DNJournal, and DNSaleprice for past sales involving that keyword. The more you look at lists of names that have sold, the better you get at understanding what sells.