How do i know which songs weren't transferred to my iTunes

Simple question.

I have 1800 songs in my macbook air.(Lets not talk about whether or not i purchased them on itunes)

When i dragged the 1800 songs to itunes, only 1785 transferred. (Where are the 15?)

In the OLD version of itunes, this was back in 2012 i think, there is a popup screen and all the other songs that weren't transferred was listed there, so all i did was copy the list and i knew what songs i needed to re-download.

NOW, there is no POPUP screen so i don't know what songs weren't included.

SO...How will i know which songs weren't synced to iTunes?

Thanks