Evernote doesn't provide any visual clue that it has been activated or that it's running in Plus mode. I *think* that the free version says that it is free in the about page, but am not sure. I've been running it for so long I no longer remember.
Hummm.... One thing that the unpaid version does is adding a kind of "slogan" or "advertisement" at the bottom of the notes you "send to an email recipient". So if you right click on a note, and select "send to an email recipient", the exported note shouldn't have any "slogan" or "advertisement" at the bottom.
That's one way, I imagine, of differentiating both versions...