How do you know if you’ve done well in a Piano exam?

I really liked this above summarised answer given by Peter Haime. To add a few more points from my own experience:

From Grade 3 and onward you also have to play ‘musically.’ This means playing (during the assessed performance time) with not only accuracy but also demonstrating the mood of the music, displaying emotion and consistency with your performance.

From Grade 5 and onward, an audience member could be expected to pay ticket money for listening. That means this level needs to be of not just a ‘passable’ standard but excellent! Often jobs in music-related industry ask for at least Grade 5 piano. Playing 3 exam pieces at this level does not cut it! You really need to be a good sight-reader and interpret music ‘musically.’

Remember to prepare for the discussion part of the assessment also. Often neglected by students in their preparation, these marks can make a difference between demonstrating that you know what you are talking about and playing or not! Examiners are not keen to give marks for discussion for one word answers. ‘Discuss’ by its very nature requires some conversational skill!