How did they check/verify bullet velocities back in the days before chronographs? When did chrono's first appear?

Early projectile chronos are mentioned as early as the 1920's.. These consisted of two discs spinning at a known RPM, and a measured distance apart..The diffrence between the bullet holes on the discs when the known distance is factored in gave the velocity..

Prior to that, the only method I'm aware of involves computing the mass of the rifle, the mass of the projectile, and while the weapon lays in a "frictionless" cradle, noting the distance it is moved aft when fired. The velocity derived is based on the amount of recoil generated, and probably was not especially accurate.

Meplat-

They also used a “ballistic pendulum”.
The projectile struck a hanging weight with a bullet-trapping medium and the movement was measured.   They also calculated backwards from the exterior ballistics.  IE – it dropped five feet at five hundred yards, therefore it must be starting out at X feet per second.
As an aside, the ballistic pendulum was used in the early days of IPSC, before inexpensive chronographs, to measure power factor.