To start, do one set of continuous, full-range push-ups to failure.
Multiply that number of reps by 2.5.
Now do push-ups with this new number as your target goal... without allowing the knees to touch the floor. You can "rest" in the top position between reps as needed, basically doing a push-up position plank or PUPP, but that's it.
So, if you can do 30 push-ups in one set, your goal is 75 push-ups (30 x 2.5). Only about ten athletes out of 100 can complete this test. Give it a shot.