You want to avoid using a working solution, like fixer or even paper developer, and pouring it back into the stock bottle. The reason is that will place processing contaminants into the stock solutions, like residual silver, etc., and you want to avoid that because those things will affect the life of the stock solution. One thing you can do to increase the life of your working solutions is to filter them when putting them back in their appropriate bottle. Even a coffee filter in a funnel works fine for that. Another thing you can do to increase the life of all your chemicals is to store them with either lids on the trays or in plastic bottles that collapse to squeeze out air.
Chemicals oxidize and air impacts their shelf life.
As for fixer itself, while there are test solutions you can mix-up or buy, that will turn cloudy when you add two drops to a batch of fixer if it's expired, when I see that it hasn't cleared a roll of film in about two minutes, I know it's time to toss it. I always use that one shot, the same with photo flo for film. Hypo clearing agent, same as fixer, a working and stock solution. Anything else, just drop me a note.