I don't know of a tool that already exists, but you can build one with the Model API - see the info here.
I personally don't have much experience with the API, but Simon Black recently posted a thread with a few example usages of the API here.
If you do create this tool (and I think it should just be a few lines of code), please share it in the linked thread!
Edit 1: Take a look here. You should be able to get a list of the activities in a microflow, find the ones that are microflow activities, and get the called microflow(s).
Jasper Van Der Hoek created a sdk script to visualise dependencies of microflows this might be what you are looking to do: https://github.com/jaspervanderhoek/SDK-mf-hierarchy