Code validators/checkers as also suggested by Bart are a handy way of picking up simple errors that are easily overlooked by eye in amongst reams of code.
And further on that note, I recently gave Microsoft Visual Studio a try and let it simply have a look at a style sheet (from the file explorer Open with Visual Studio, no need to create a project). I must say, it really gave useful suggestions when the WebEssentials extension is also installed. Sometimes suggestions as to 'Best Practice', sometimes useful (intelligent) additions for cross-browser compatibility, sometimes an error. Of course wen part of a project, VS can do much more like showing the direct impact on a webpage with a preview, but merely as a code checker it also works fine.
Cheers,
Bart
