Add a version.json file to the web root folder with two elements:
ModelVersion = Core.getModelVersion()
RuntimeVersion = com.mendix.core.conf.RuntimeVersion.getImplementationVersion()
You can then check the version of the running app by just calling the endpoint https://YOUR.MendixApp.Domain/version.json
Currently i am creating such a file by using an OnStartup Microflow and a java-action.
If you run your app outside the Mendix Environment you somehow need to know which version of your app is currently running.