What would classify those findings of the University of Sheffield experiment as aliens?
I wouldn't say that there is an established protocol - claims of alien life are not commonly testable or even legitimate - but if something were discovered and deemed "alive," a fairly good sign it was alien would be if its genetic makeup were based on something other than DNA or RNA.  That would be a dead giveaway the organism had side-stepped every known organism on Earth and while that doesn't prove alien-ness, it would certainly hint strongly of a non-Terran genesis.  There are arguments for and against carbon/CHNOPS-based life but that is nowhere near as definitive.

Also, there's another really good technique.  I don't like to resort to ad hominem attacks, but ideally the discovery should be by someone who has not already made verifiable false claims on the same topic.