As you know, film and the chemicals of the developing process all have a shelf life.
As to the chemicals of the process:
Concentrates are packaged photo developing chemicals that must be diluted with water. Concentrates, in the original packaging, unopened, have an indefinite shelf life, perhaps many years.
Stock solutions are concentrates that have been diluted with water. They are too concentrated to be used and must be further diluted with water at the time of use. Concentrates are generally good for about six months to one year - provided they have been stored in capped nearly filled bottles.
Working solutions are the chemicals of the process that have be diluted with water to working strength. If unused in and stored in capped bottles, nearly full, they have a shelf life of about 2 months.
Used working solutions can be stored after use. Best to use plastic bottles and squeeze them as you cap to expel all the air above the liquid. One trick is to add washed marbles to a partially filled bottle this will reduce the amount of trapped air. Used solutions should be discarded after one week. Keep track of the number of films developed. You likely must increase the developing time according to a schedule included with the packaging.
Replenished working solutions are photo chemicals that have been treated with a chemical formula that rejuvenates. If the replenisher solution has been stored properly with the correct amount of replenisher added, they can be used indefinably provided the solution is frequently used and replenished properly.
Developers when freshly mixed are clear fluids, usually the color tea. Their main enemy is oxygen. Developers are cousin to benzene, an off-shoot of coal. As the developer solution ages, it takes on oxygen and reverts to coal tar. Thus spent developers turn dark, even black. The developer formula contains a preservative. All the while the developer is on the shelf, the preservative is working. After a time the preservative exhausts and the developer solution has no protection.
We can truly only test a developer by means of a chemical test or by developing a sample strip of film and examining the results. You can, in the light, test by develop the sniped off tongue. It will turn black before your eyes. This will be a limited test, use this fluid with caution. If in doubt, best to discard and mix afresh