They keep surprisingly well if cold and not exposed to oxygen, they can decline at rates of <10% per year in those conditions so don't assume they're knackered. Let your nose decide - the cheesiness test is a good one.
Having said that, aside from the usually advice to use hops that have lost all their alpha acids in sours/lambics etc, old hops can be used as "enhancers" in the whirlpool of hoppy beers - it's all to do with hop acids oxidising to branched-chain fatty acids such as 2-methylbutric acid, which can enhance the "tropicalness" of other hops and also esterify during fermentation to esters such as 2-methylbutyl isobutyrate (2MIB).
See Scott Janish's book, or he touches on it in this article :
https://scottjanish.com/survivables-unp ... op-flavor/
