https://en.wikipedia.org/wiki/Galactic_year
“The Solar System is traveling at an average speed of 828,000 km/h (230 km/s)”
1 / 299792458m = 0.0000000033356 sec (time it takes light to travel 1 metre)
230km * 1000 = 230000m (Solar System average speed around the galaxy in metres)
230000m * 0.0000000033356 = 0.000767188m = 0.767188mm (distance Earth moved in orbit around the galaxy in the time it took light to travel 1 metre)
If light travels at only c and not c + v (where v is the velocity of the emitter) then we should measure a drift of 0.767188mm over a 1 metre distance and a 230km drift over 299792458m.
Take a mounted laser and shine it against the wall from a distance of 1 metre. Draw a dot on the wall where the laser light is and keep it on for a year. If light travels at only c then we should detect a drift of up to 0.767188mm from the dot we drew on the wall. The Earth is spinning and orbiting the sun so as it spins the drift will change direction depending on the direction we are facing relative to the galactic year orbit direction/speed of 230km/s. If the orbit speed is to our left we should detect a drift to the right of our dot and in 12 hours when the orbit speed is to our right the drift should be to the left of the dot.
If we don’t detect a drift then light travels at c + v