On RHEL, assuming your system is registered with RHN, all you have to do is use “up2date -u” or “yum update” (depending on major version) to install latest patches. up2date has the “–dry-run” parameter that simulates the update without actually doing anything; that will give you a list of missing, updated packages. yum has a command/option called “check-update” that does something similar.
On AIX, you use the compare_report utility to generate a list of installed LPP filesets and compare it against a list provided by IBM. I use SUMA to actually download (which can also do the compare in one go) pending updates; it can do APARS, individual filesets, tech levels, service packs, the whole gamut.
For Solaris, I normally just grab the latest Recommended patch cluster and run the install_cluster script in the archive; any missing patches are applied automagically. I know that there are some first- and third-party compare scripts that will do something akin to IBM’s compare_report, but I’ve never bothered with them.
I can’t speak for “hockey-pucks” as I’ve never had the displeasure of using it.