How does one go about figuring out a 500 error?

[Antti-Juhani Kaijanaho]

My Planet (running PlanetPlanet 2.0, though Venus seems to have the same problem) consistently
gives me "Error 500" on several feeds.  That is quite uninformative.  How do I figure out what
the problem actually is, and whether it is at my end or at the feed's end?

(Assume I have no way to change the Planet code, and you'll be fairly correct.)

-- 
Antti-Juhani Kaijanaho, Jyväskylä, Finland
http://antti-juhani.kaijanaho.fi/newblog/
http://www.flickr.com/photos/antti-juhani/