Version-related reconnaissance
Looking up versions, release dates, and related information.
Finding a version's release date quickly
Search Google for: name version changelog
For example, to find the release date of pfsense 2.1.3 and compare it with patches, exploits, and so on, search for:
pfsense 2.1.3 changelog

A technique for spotting vulnerable versions that I haven't used much and still need to learn
Compare the current OS version and the service exposed on a port with the corresponding service shipped in that OS release, to see whether the current service is an older version.
For example, compare 4ubuntu2.2 + apache 2.4.18 with ubuntu2.2 + apache 2.4.1x at release. Something along those lines. I'm not clear on it yet, so I'm writing it down for now.