> Is a past participle a verb or an adjective in the passive?
The Part-of-Speech guidelines for the Penn Treebank, a corpus annotated for part-of-speech and syntactic information, might be of use here:
http://www.cis.upenn.edu/~treebank/
ftp://ftp.cis.upenn.edu/pub/treebank/doc/tagguide.ps.gz
page 17, "JJ or VBN" "The distinction between adjectives (JJ) and past participles (VBN) is often very difficult to make. There are a number of tests that you can use to decide..."