E6 FD diluted 1+9 with tap water makes an excellent black&white film developer, especially for slower, fine grained films. However, you'd have to establish a base line to see, whether a specific batch of E6 FD is still fine. If you don't have a base line, or a small sample of known good E6 FD to compare against, then your best bet is checking the color of your E6 FD concentrate. Anything darker than pale brown will show lower activity, which can sometimes be compensated by using longer E6 FD times. YMMV.
The best investment so far were these two boxes of Fuji process control strips for C-41 and E-6. They are pricey, I grant you that, but so is a roll of film going to waste because of some mixing error, not even talking about the loss of subject matter.