With a chrono you can test the FPS of your replica. We have them from brands like XCortech, Union Fire Pro, Guarder and Madbull.
Union Fire Company
Union Fire Pro Chronograph Desert
Xcortech X3500 Chrony
Guarder Speeder 2000 Gen III Shooting Chrony
FPS/MPS/RPS and Joule
Madbull Version 2 Chrony