A study meant to reach a consensus between veterinary experts in feline medicine on the core signs sufficient for pain (sufficient to indicate pain when they occur, but not necessarily present in all painful conditions) and necessary for pain (necessary in the presence of pain, but not always indicative of pain) was recently published.
The study consisted of four rounds of questions and evaluation with nineteen participants. Agreement was considered to be established when 80% of the experts concurred with the same opinion.
Twenty-five signs were considered sufficient to indicate pain, but no single sign was considered necessary for it:
These signs may help both vets and owners perform an initial evaluation of the pain status of cats in their care.
Via: http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0150040