There are a few easy ways to tell whether you are doing an exercise correctly. 
Are you feeling it in the right place? Does the exercise that is being performed fatigue the required muscle group or is the effort coming from another muscle group? An example of this would be neck pain during a set of abdominal crunches which indicates that the form is off.
Are you using the full range of motion? Full range of motion is an important part of proper form. Each exercise should be taken through the complete range of the joints movement controlling the weight through the entire range of motion. This means while performing an exercise. An example of this is the half squat. Instead of only partially going down, lighten the load and perform a squat until the thighs are parallel to the floor.
Have you got full contol on the weight? Using momentum to complete the movement increases your risk of injury. An example of this would be bouncing the Barbell during a Bench Press. The movement should be controlled and should be able to pause for a brief second at the most difficult part of the movement.
PLEASE ASK US AT THE DESK IF YOU REQUIRE ASSISTANCE TO MAKE SURE YOU ARE USING CORRECT FORM