Just try to remember that the tester doesn't always know what you're doing or what you can see so you have to make everything especially obvious for him or her. Once you've past your test doing what you did was fine, you knew nothing was there because you'd been checking in your mirror so it was impossible for anything to have got into your blind spot without you seeing it BUT your instructor doesn't know that you was checking or what you could see so as far as theyre concerned you didn't look. a quick check of your mirrors in normal driving is fine, most of the time you don't even need to turn your head much you can just move your eyes to glance in them but again in your test make a big deal about moving your head to the side so the instructor can see what you're doing. Those who pass first time aren't necessarily any better, more often than not they were just luckier.
One tip I was given is move your head to look at your mirrors to check them. It makes it clear to the assessor that you're checking them whereas if you just glance with your eyes it's nigh impossible for him to tell.
I passed second time and it's a well known, scientifically proven fact that those who passed second time are the best drivers in the world, ever.
I wasn't sure about when to check on my test so I just did whenever pulling out, whenever starting moving when in lanes of traffic, or random other times where I thought it might be a good idea. I'd say if in doubt, check. After I passed I can't remember exactly what my examiner said but it was regarding checking too often and that there were several occasions where I didn't need to. I didn't really listen because I was just relieved to have passed. She said you can't be marked down for it though.