Checking Who has a Document "Checked Out"

Hi!

I work for an organization that needs to access PDF's often. There are few files I've needed to edit that I can't because they're "checked out" by another user. Unfortunately, it doesn't tell me who the user is. We have high turnover and I haven't been able to access these files in a little while now, so I don't think it's just somebody who's had the file open for months now. I was hoping there was a way to see who has documents checked out - or even just a list of checked out documents so I can stop trying to open the ones that I can't edit. 

Based on google searches all I'm really seeing is something about green check marks next to the file/right clicking to "check out" but none of those options appear for me. It just gives me the error that someone else has it checked out when I try to open it. 

When I close a document I have checked out it asks if I want to check the document "in". I don't know what happens if you hit no - so, it's also possible staff have just accidentally hit no and not realized they never checked a document in. I want to ask them to check what they have checked out but I can't instruct them on how to do that if I can't even figure it out myself. 

Hoping someone can provide some clarity! Thank you!