You can call Get App Instance (REST | SDK) to retrieve the internal version of your app that is installed on each Wix user's site. The response contains app information:
It also contains site information:
On this owner's site the app’s major version is '2' and minor version is '10'. Wix users will get minor updates automatically, to 2.11, 2.12 and so on. However, the next major version, 3.0.0, will require the Wix user to click the Update button in the App Manager or follow the instructions in your email or banner.