With a chrono you can test the FPS of your replica. We have them from brands like XCortech, Union Fire Pro, Guarder and Madbull.
XCortech X3200 Mk3 Shooting Chrony
Acetech AC6000 Chronograph
Guarder Speeder 2000 Gen III Shooting Chrony
FPS/MPS/RPS and Joule
Madbull Version 2 Chrony