It is easiest to see the difference between fresh grit and 'spent' or 'broken down' grit with stage 1 (60/90) grit. When you put new grit in the barrel, the grains will be relatively large and easy to feel the grittiness with your fingers. Once it is spent, you will no longer see these larger grains; you will instead likely see a fine sludge at the bottom of the barrel or cleaning bucket. If you still see larger/grittier grains, then you are probably adding too much. This is not a problem, it is just wasteful and more costly in the long run.

I clean mine using a sieve/strainer (like this one) over a 5 gallon bucket (like this one) with a hose or other water source. I then dump the dirty water into the dirt/grass. Look at the contents of the bottom of the barrel when dumping it to see if there are unspent grit grains left over.