How to inspect and validate a deb package before installation?
Use this:
dpkg-deb --info <deb file>
Prefer to use apt if you need them to be signed.
Normal deb files do not contain all data you need, except that you can get by dpkg-deb --info or look into DEBIAN/control file.
You can have dsc files with this data, if you download from launchpad or official repositories.
Deb files are not signed by default. General recommendation is not to install deb packages from sites you do not trust.
There are no special security instruments in Debian packages.