Every now and then I get a gig where I go in to a customer to help sort out an exiting problem with their SharePoint environment (get us engaged early in the projects folks, that eliminates quite a bunch of these panic engagements). When trying to get a grip on a existing environment I use a couple of tools, one of which is ULS Viewer. (Then name says it all :)).
Jie Li has a good write-up on using ULS Viewer for trouble shooting over on his blog, check it out.
(And of course it’s the official documentation available at http://code.msdn.microsoft.com/ULSViewer)