This mod shows inserter throughput on hover. There are options to configure displayed precision or to temporarily disable the mod for yourself.

The throughput value is calculated based on the selected inserter's stats, pickup and dropoff positions and entities. This means that it works with most modded inserters and belts (in case of a belt being involved). For chest-to-chest and chest-to-belt the value is precise. For belt-to-chest the value is approximate.

The inserter measuring tool can be activated with the /it-stopwatch command. It will try its best to determine throughput based on activity, so make sure your inserter is constantly working if you want to measure maximum throughput.

If you want to contribute to this mod, any information about inserter mechanics of picking up from belt will be helpful.