Fixer is pretty cheap, so use fresh solutions and remove all doubts.
For Film, you should use fixer at working solution concentration. You can test it by putting the film leader (that you cut off) into the solution and wait for 2 to 3 minutes. If the film clears up, the fixer is OK. If it looks somewhat murky, the fixer may be close to exhaustion. An alternative technique is to use fix the film twice via first using an older fixer, then a fresher fixer (many articles written on this). My personal habit is to develop a batch of films at one go (about 6 to 10 rolls) - and only freshly mixed chemicals are used, which are then disposed after the last roll.
For paper print, the fixer can be used at lower concentration (as compared to film). Prepare enough fresh fixer for the day's printing. At the end of the day, dispose the fixer.
It is not a good practice to risk incomplete fixing of film or paper by using the fixer to it's max capacity. Problems related to incomplete fixing may not show up until many months or a few years later.