If you've downloaded any content from PSN on your Vita before now, it's activated. If it's a used one, if it has any installed games when you got it and you haven't done a restore, it's activated. If you've done a full restore and haven't connected to PSN to download something before PSN spoofing went down, you're likely unactivated.
Edit: Same even if you copy act.da
If you don't know either way, I don't believe there's a sure-fire way to check anymore but you can go to Settings-> Playstation Network to see if there's an account associated with the device. Make sure you're in Airplane mode while you do this, otherwise it will just ask you to update.