Install debsums
sudo apt-get install debsums
Debsums will verify the files on your system against the files in the package repositories (and tell you if they differ)

Most packages include the neccessary info to verify them against the repositories, but some don't, so first up, run
sudo debsums_init
This will generate file hashes for any packages that lack them (note, this involves downloading the package from the repositories)

Then you can run the verification
sudo debsums -c