You can't tell if it's previously opened because the way they make custom paint colors is they open the can of paint and add colors to it and mix it up so when you buy it, it's already opened. I guess you could check the paint level to see if it's full. I don't recall how much cheaper but it was substantial enough to make it worth while (in my opinion). It's usually only good if you need small quantities as it's hard to get the exact shade if you need them to mix up more.
