Is there a such thing as perfect condition? Surprisingly, the simple answer is yes, especially for those polymer notes. Paper note is harder, as many of these come with at least the cutting cup mark located on top of the edge where the security thread is in bedded or some biding marks on the top and bottom edges.
Consider using this simple technique.
1) Hold the note flat at or just above your eyes level at an
angle of say between 20 to 40 degrees angle;
2) View the note surface with strong light. Natural sun light would
be the best. Table lamp light is still good but has to be close up;
3) Move the note slowly, like chancing the angle of viewing
between 20 to 40 degrees. The lights reflected on the note will reveal any flaws on the note. Here you should be able to spot any folds, creases, teller flicks, dent
marks (both paper and polymer), and even the original condition of the note material
Try it out yourself. It's very simple.
A note may looks uncirculated condition to the naked eyes,
but until you try the above technique, you can't be sure. This is on top of
checking all the corners of the note. Cutting cup mark is very common nowadays
and can be spotted by just simply looking at the note. Same goes to binding
marks too, especially along the edges.
If you have a brand new note and it looks a bit wavy (not the edges), that is okay. Imagine placing some liquid on a piece of paper, it bounce to go a bit wavy. Place the note in a sleeve and lay in flat for a while and in no time you will see those wavy effect disappear.
Remember that a Polymer note can not be repaired once damaged. Once
folded or creased, there is no way you can repair that. Don't even try pressing
it with hot iron. It can only made the note crumby and shrink in size, after all it's plastic. If you have been offered a paper banknote in prefect condition, ask the seller if the note has been pressed.