There are way too many factors and unknowns that you would have to consider to simply calculate the thrust. The simplest way is to do it empirically - build a rig where you can simply weigh the motor, subtract its weight and start increasing rpm and measuring the thrust in steps and also current and voltage so that you can calculate efficiency. The most important factors that will impact the thrust and efficiency are: the type of the motor, the propeller pitch and diameter and the materials the propeller is made of and how well the motor and propeller are balanced. Generally the motors that have lower RPM/V (KV rating) and can run on higher voltages are more efficient because higher currents mean higher heat loss.
Keep in mind that even with the rig with the scale, your measurements won't be 100% representative because first of all the motors are attached to arms which create drag and energy loss due to non-zero stiffness and when you have several motors running in the same plane next to each other, they interfere and the actual thrust is slightly lower. The closer the tips of the propellers will be to each other, the less efficient it becomes. A recommended separation is at least 1/3 of the propeller diameter.