How to check number of installs on PSE 15 serial number?

Hello!

I purchased PSE 15 on amazon.com via install disk, and it appears like the package may have been opened before. How can I check to see how many installs have been used on this serial number? It installed just fine on my Mac (osx sierra), however, with the fishy packaging, I wanted to see if I indeed still have one install left. I don't want to use it just yet, so how else can I check?

Your help is greatly appreciated!