Tim,
Until CS5, registration was not required (from CS5 on, registration formed part of activation), so an older version may not be registered.
If registered, a licence transfer through Adobe is required in order to obtain legal ownership. If unregistered, the previous owner can hand it over quite legally, if it is legal in the first place.
Hopefully, there is a viable serial number/licence key somewhere; unfortunately, such information may get lost, but still everything may be legal. It may be needed later in case a reinstallation is needed (maybe on another computer) because a new non activation version has been made because the old activation servers have been retired (you will need the (old and legal) serial number in order to download).
Whoever owned it before may be able to find the registration number somewhere, or if registered find it on this page under her/his (then) Adobe ID:
https://accounts.adobe.com/products
Failing everything else, you may ask Adobe in a chat here:
https://helpx.adobe.com/contact/support.html