Malt crush
Take a 8 oz sample of the malt.
Mill it
Weigh it and make sure it is still 8 oz. Look for places in the mill that might be collecting grain and clear them.
Don't shake it or compact the sample. Ideally, it should be as fluffy as it comes out from the mill. Sieve the crushed malt through. Capturing the dust is a plus, but not required.
Use a fresh paint brush to remove any particles that went through the sieve but clung below the screen.
Carefully pour the remaining malt into a clean container to weigh. Make sure to tare the scale to the container.
You should have between 5.2 and 5.6 oz of grist. This represents 65% to 70% of the grain weight. Above that and the mill gap is too wide, below that and it might be too fine. The number in a pro brewery is 50%. This is pretty perfect for a homebrewing setup and shoots for 85% total mash efficiency when fly sparging.
Secondary, you can sieve this through a #4 screen to separate out large and uncrushed grains. This weight should not exceed 5% and is hopefully just 2%. Look for out of parallel rollers as a source of large sized particles. This may not be correctable since at least one manufacturer (Schmidling) purposely engineers their mill to have a varied gap.
Obviously, you'd repeat the above until the crush lined up with optimal settings.