Testing for non-ascii characters copied from webpages

So, I'm finding lots of things about removing non-ascii characters, but not really adding them.

Basically, I have a text field that a user can type in, and then that string gets processed, stored, and presented under certain contexts. I expect the user to sometimes just copy and paste text from other webpages, and I want to make sure that nothing the user enters in that field will break anything. (I know this is a potential problem because a user coping and pasting a ' that was not actually an ascii ' already broke things once)

This is NOT about removing non-ascii characters! I want a good list/file of possible problem characters I can copy and paste to verify that they get processed correctly. Or at the very least, a good way to find these potential copy paste 'impostor' characters.