How do I know if an applicant is going to blow a check ride? There has been one reoccurring clue and that is the applicant has some sort of self imposed deadline. For example, leaving on vacation, vacation ends, school starts, moving, instructors leaving, surgery scheduled or written test expires.
It doesn't always happen but it happens frequently enough to spot a trend. Somethings get rushed or skipped in the training process and the stress from the upcoming check ride combined with the self imposed deadline bundles up to be a little too much.
So if you find yourself in this situation ask yourself can the check ride wait? What if you bust the ride, you'll have to retrain and retest that will take even more time. Personally, I would rather see you wait a couple weeks or a month rather than see you bust. Your check ride performance will improve 1000% when you don't have that additional stress!
I have an open door policy on my check rides. If an instructor wants to sit in, as long as it's ok with their student, they certainly may. I even encourage riding along on the flight.
There are always 3 sides to a check ride; what the examiner sees, what the applicant sees, and what really happened. Sitting in on a ride will give you a much better pitcher of what is really going on.