The version is not embedded in the Spine Runtimes code. You could look at the commit dates and find May 1, 2019, but I wonder where you got that date and if it would really be the exact commit you have. Short of checking out versions from Git and comparing with the Spine Runtimes code you have, there isn't a way to know the exact Git commit that you have. However, this is likely not needed. You can look at the version of the exported data, as Erika described, then use the latest runtime that matches that version. For example, if your data was exported with 3.6.20 then the 3.6 Git branch has the latest runtime for your data.
An alternative would be to export your projects with the latest editor and update your app to use the latest runtimes.
I see your additional post now. The latest 3.6 (for example) runtimes will work with exports from any 3.6 version of the editor. If you data was exported with very different editor versions, eg 3.5 and 3.6 then you should export them with the same editor version (at least major dot minor version).
If you don't have the project files for your data exports, you can import them into the same version of Spine they were exported from, then save a project file. You can then open that project file in any newer version of Spine. If you have many projects, you can do this from the command line:
Spine 3.8 released - mesh tracing, polygon packing, and more
