I have all my songs in a folder, when I need to add songs I do "Add folder to library"
Now I have deleted some songs from my folder, how do I tell iTunes to check which songs were removed and remove them from my library to remove them from my iPod?
iTunes has no functionality to actually scan for missing songs from it's library. If you want to delete Music, you need to do it from within iTunes, and not the other way around. It can however identify missing files which normally appear with an ! mark next to their names. These can simply just be deleted out of the library manually.
There are some 3rd Party options to try and do this automatically:
Once you have removed the songs from the iTunes library, they will be removed of your iPod the next time it synchronizes.
I've found the easiest way to do this, detailed in the answer here :
No need for third party tools.