Tool / utility for checking debugging XPath queries

SketchPath looks like it might help (and is free)

If you have Notepad++, there's an XML tools plugin. Among the tools in that plugin is a simple XPath expression evaluator. Sketchpatch is much more complete (I've tried it and it's nice) but if you use Notepad++ all the time it's worth knowing about this plugin.

There is a new plugin for Notepad++: XPatherizerNPP. The biggest advantage it has over XML Tools is the reverse lookup feature.