There are two kinds of brake booster, both can be tested while parked,
Vacuum boost: press and release the brake pedal several times the brakes should be very hard.. Now while holding the pedal start the engine, Pedal shold move toward floor and feel more normal.
Hydro boost has two ways it can be powered, EITHER an electric pump or by the power steering pump.
While parked, Press pedal, You should HEAR the pump motor start and the pedal will first stop or seem to then continue SLOWLY by comparison to go down part way to the floor.
With the engine running it should go down much faster.
And that's all there is to testing them... From the driver's seat.
NOW: a proper technician with the needed tools may be able to do a more "In depth" test.. but that is the basic test.
Home is where I park it!