I have found the best indicator of bad reagents is when the results don't make sense...i.e. the results you get are higher than expected (reagents starting to weaken).
If you are still getting consistent results, then they are likely fine.
I have had, even though stored in cool, dark place, reagents go bad before expiration. Salt test being one of them.
My best practice is to order enough reagent for a year, and replace yearly to avoid any doubt.