Drone crashing when i try to fly it

my drone is toppling to one side and crashing instead of hovering. i have a feeling that one motor is rotating at a lesser RPM compared to the others. How do I check the RPM of each motor? I know a laser tachometer can be used but i don’t have one. Can I get this information from Mission Planner?

Unless the motor does not move freely the RPM of the motor is controlled by the ESC’s. That is why they are calibrated. The flight controller could care less about the RPM of the motor only that if it leans to the left it will spin up that side to level the copter.