Fire the gun several times, changing the angle of release. Manually measure some angles with a protractor. Fire the gun again. Pick the gun up one meter off of the ground, but keep the same angle, and shoot. Keep the angle and height of release consistent.
Most dart guns have a set speed of release.
As an example, try moving your arm forward as the you pull the trigger. The resultant initial velocity a dart leaves a dart gun is the speed of release. This is accomplished by determining the initial horizontal and vertical components of velocity, while using different angles of release. The height the dart travels at apex of flight can also be determined for each angle. Record the following values for each dart's flight.