Heat your hotend to printing temp. With no print running, use your slicer or printer menu to extrude exactly your target length (default 100mm). Before extruding, mark the filament at a reference point on the extruder entry. After extruding, measure the distance from your mark to the entry point — that gap is your actual extruded length.

Found in printer firmware or EEPROM (M503 in Marlin)
How much filament you commanded the extruder to feed
How much filament was actually fed — measure with calipers

Calibrated E-Steps
steps/mm