On Sharp, and on the color Kyocera machines you can run a simulation to agitate the developer and give current toner density readings.
For Sharp it's sim U25-01 (typically around "100")
For Kyocera it's sim U131, Automatic (difference of less than 25 points between reference and actual)
I'm sure Ricoh has their own counterpart, it's just been too many years.
Visually if you remove the developer unit and rotate it manually, overtoned developer cannot hold all the toner available, so you'll see a cloud of loose toner wafting off the developing roller while agitating. Undertoned developer will either leave you gritty copies, piles of developer beneath the mag roller, or bare spots in the developer brush.
Count is a very good, reliable reference. Perhaps the most reliable.
Just looking at image quality? You can see the same effects from developer, drum, primary, other things ... =^..^=
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