Got a trip planned and curious about van Goghs in the area? The splendid site "Vincent van Gogh Gallery," hands-down the best site on the web for all things Vincent, actually has a World Map of Van Gogh paintings: http://www.vggallery.com/map/main.htm. This past spring, I was set to give a talk at the University of Kansas, and I went straight to the map to discover which van Goghs the Nelson-Atkins Museum had...then contrived to get myself there. It's become a little ritual of mine with trips to new places.
It gets trickier when it comes to the museums themselves: how to know if an artwork you want to see is out on loan? It's a sad, sad feeling when you arrive at a museum gallery and hunt in vain for The Special Painting you've been longing for. Unfortunately, most museums don't have this information readily available, but some do. The Musee d'Orsay's new site has a nifty floorplan which lets you know what's hanging where in the museum (and they seem to keep it updated). The Kroller-Muller Museum in Otterlo has a cyberlist of artworks out on loan to help prevent unpleasant surprises. But most of the time--you're taking your chances. Happy Hunting!