Some days ago I totally destroyed a 135 film developing it with a dead chemical (it was a solution of R09 done from an already opened bottle. See here). It is not a nice experience, for sure. So I started to search for a practical method to check if the developer is still good or not. This is what I found and I tested it by myself.
You can do the whole stuff in full light.
STEP #1: Put a small piece of the film on a toothpick (part of the film leader that usually you cut away before putting the film in the tank roll) and put it into the developing solution. This film has been exposed to full light so it should become totally black. Keep it in the developer for 1/2 or more the time you need to develope the film.
STEP #2: wash the film under water
STEP #3: put the film in the fixing solution and wait for 1/3 time of usual fixing time
Now, if the developing chemical is still good, you’ll get a total black film. If the developing chemical is gone, you’ll get a totally transparent film (fixing solution took away all undeveloped silver).