The easy answer is 'You'll Just know'. When the student is capable of carrying out the tasks you set, to a standard you set yourself (obviously not at the expense of the student and lining your pockets), without input from yourself to correct for mistakes, it's then for the examiner to pass or fail him. If the examiner knows you well enough, he'll know that the student you're putting up for test should, under normal circumstances, be capable of passing the test. It all comes down to experience. When I first started sending student ppl(h)'s off on their first solo's, I was a nervous wreck.
For a ppl test, the student should be able to demonstrate the aspects of the course within defined limits, without your input, and hopefully have the mental capacity available to carry out other tasks, ie map adjusting etc You don't want to overload a ppl student too much but it's good to give them more and more stuff to do. Make their brain work hard.
It's a steep learning curve, enjoy the ride.
