These circuit contraptions split a belt into lanes and check whether each lane is perfectly compressed and moving at max speed (not backed up). A version for each belt color (yellow, red, and blue) is provided.
For each lane a lamp shows its current status: red if it's does not have max throughput (either backed up or not enough items), yellow if it currently achieves top throughput but there have been issues since the last reset, and green if throughput has been perfect since the last reset. To reset the setup, toggle the top left constant combinator. Doesn't matter if you toggle it on or off.
When resetting multiple checkers, toggle one constant combinator by hand, copy its settings with Shift + Right click, then paste its settings to reset combinators in all the other checkers with Shift + Left click.
Hover over the arithmetic combinators in the middle to see for how many ticks throughput was imperfect since the last reset - it's the value of the yellow signal.