I am doing a science project on how paper airplanes generate lift, and I need to know how to measure the amount of lift generated by the plane.
When you throw a paper airplane and it flies in a more or less straight line, its lift is equal to its weight, because it is traveling in a straight line. So all you have to do is weigh the airplane.
The more tricky problem is to know its velocity and it's angle of attack.
Lift goes as velocity squared, for a given wing area and angle of attack. So, for example, if you reduce wing area by a factor of four, you will increase velocity by a factor of two, to fly in a straight line, at a particular angle of attack.
The angle of attack is controlled by the elevators at the tail of the plane. If you angle them up, the plane will be at a higher angle of attack, so it will fly slower to get the same lift.
What you need to do is 1) measure the weight of the plane, and then 2) determine the relationship between wing area and velocity to achieve straight-line flight, for a given setting of the elevators.
Read this site. It's great reading and will really educate you about flying.