What hierarchies are actually being used in my cube?

Working out how to monitor Analysis Services usage is something I've spent a fair bit of time doing over the last few months; I'll get around to writing up my SQLBits presentation here soon, I promise, but in the meantime here's a quick blog entry on a related topic: how can you work out which dimension hierarchies your users are actually using in their queries? It's useful to know this for a number of reasons, for example if you're thinking of changing the AggregationUsage, AttributeHierarchyEnabled or AttributeHierarchyOptimizedState properties. Or if you're just plain curious.