Here is how to tell if art is truly amazing. This is the way I see it all anyway.
To dub art amazing you need a King, Queen, Earl or Duke of a creative realm to do so. Actually it just takes an opinion of one person or more. When art strikes someone in that emotional way were there eye can not leave it and an unexplained excitement develops within them, that dubs the art amazing. They may even say to themselves "Wow".
Because you put your all into it and really emotionally cared about what you were creating, makes it possible to be amazing art. With years of practicing and experimenting with media and surfaces as well as subject mater, there can be a crossover of talent, technique and subject. A crossover that makes a special connection with the viewer. As an artist you may only see the struggle to get to the end result and dislike what you have created, but the viewer only sees the art as a fresh creation.
Not sure what being "unique" with your art really mean. I guess it would just be you being honest and not straight-out copying another artist or style. Whatever "unique" is, we as artists strive for it and feel we need to find it, like it is a hidden treasure. Being "unique" is a key feature as well in the creation of amazing art. There is no road map or instructions that can get an artist there. We are so different in our creative process that it really is ridiculous to think there is only one road to get there. I feel that it is just something that happens at a certain point after countless art creations and experimentations. Not sure that the artist really even knows when that happens. If only a magical owl could shower you in notifications like in Harry Potter. The fact is, that we all are by nature unique and amazing. I find that shooing away negative thoughts is a must for staying on the creative path.
So, in conclusion, when a viewer makes a special connection with some art and is in awe, the Earl of amazing art dubs the art to be amazing and you will be showered with notifications of the such. Henceforth, that art is amazing! But, just know that that does not make everything you continue to create, amazing. An artist must put just as much heart and soul into each and every art creation to make them amazing as well. It really is a healthy goal, but if it is not reached, it is okay, you created something that did not exist before and that is amazing.