According to PPH:
"With proper storage, the sodium salts have an almost indefinite shelf life"
i recently tested my 3yr old SN and blood turned dark brown immediately
Just to piggyback off of this.
Whatever the item in question is:
1: Make sure it comes from a trusted source.
2: Leave it in whatever container it shipped in.
That means if it is shipped in a vacuum sealed bag, do not break the seal until ready to use.
If the item is shipped in a container, do not break the cap seal until ready to use.
etc.
3: Keep whatever you're storing in the coolest, darkest place you can find.
4: If you must test them, do so on the day of use or the day before, but never on yourself.
I might be wrong, but there are numerous methods for testing nearly everything mentioned here that do necessitate someone being a guinea pig.