How to verify installed files?

[Erik Persson <erik-maillist@djingis.se>]

Hello,

I'm trying to figure out how to verify the files installed from a 
package by apt-get, ie I have installed a package A and want to verify 
that the files installed from the package are correct at some later 
time. Thus some process could have changed some of the files after I 
installed them, so the checking that is done at install time is *not* 
what I'm looking for.

A related question. If I want to check that a package (in the archive - 
/var/cache/apt/archives) is correct compared to the same package located 
on a repository. How do I do this in the simplest way. I assume that 
md5sum is correct.

Thanks in advanced,

Erik.