Thanks to the Web, there are several sites that will educate you about the publishing industry. It's up to you to read their content regularly and do your homework on what's selling and what's not - and why.
Of course, you need to do your footwork, too. Check out what's being touted in the grocery store's book section, as well as WalMart's and Target's book aisles. Roam through there and see what people are wanting to read. Do it regularly. It goes without saying that you're also going to be doing this at the local bookstore. Talking to the worker bees there - "what's hot this month?" - can be very enlightening.
As for the sites, there are a few that attempt to do this forecasting/prediction business for you. Publisher's Marketplace, for one. BooksInPrint IntelliMarketing looks pretty, too.
However, as one of my famous, very successful, author role models recently warned me, "remember whatever wonderful things that are said are simply whatever the publisher or agent or whoever sent to PM. It isn't exactly unbiased."
And, there's another negative: they're not free. PM costs around $250/yr for the privilege of accessing its site information, for example. And, you have to email BooksInPrintIntelliMarketing in order to learn its cost. Now, that's skeery.
So, we come back around to the free web sites. Here are a few:
WriteNew's long list of industry links
PublishingIndustry.Net's MostRead Links Page
USATodayOnline's Book Section
ParaPublishing's Collection of Statistics - a must read
Yahoo News on Books/Publishing
Google News - nice to create a news alert here for your particular interest - say romantic suspense, or cozies