Well, one way of getting credits for vocal albums (and fairly reliable) is Anison, entry for NARUTO SUPER HITS 2006-2008 as an example:
http://anison.info/data/source/34511.html
(most important columns are 3-6: performer /singer/, lyricist, composer, arranger)
then some online shops have such details as well. Scans are always the best, but when they're not available, stuff like this works.