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The closest thing is probably revdep-rebuild.
It checks if any packages are broken due to missing or changed libs.
emerge takes care of package dependencies pretty much automatically so you should not really need a separate tool to check if something is missing.
If you happen to uninstall a package that is needed by something else then revdep-rebuild would tell you and the next "emerge world" would pull that package in again as well.
That's what I can think of right now but there are probably different ways to do this.
EDIT: But I don't think there is anything that checks if the installed files are really still there, except for libs. Edited 2007-12-16 12:51
And for Paludis users, reconcilio is the revdep-rebuild equivalent
To me it sounds like a combination of revdep-rebuild and emerge --pretend depclean (don't ever forget --pretend).