Which file is my reflection probe using?

Unity 2018.4 I have a reflection probe component on an object in my scene that's used by several mesh objects.

How can I tell which cubemap (either auto-baked or manually baked) is in use by the probe?

And how do I set the cubemap for my probe - do I have to use a probe type of custom for that?

In particular I'm trying to export a scene from a project for use in another project, and it seems like the only thing not coming through in my asset export is the reflection probe's cubemap.

Thanks for any advice!