I'm note a programmer. I Don't Understand Codes. How do I Know If An Open Source Application is not Stealing My Data Or Passwords? Google play store is scanning apps. It says it blocks spyware. Unfortunately, we know that it was not very successful. So, can we trust open source software? Can't someone integrate their own virus just because the code is open?
In terms of telemetry, free software has the advantage over the proprietary counterpart.
It's a lot more complicated to hide telemetry without the user knowing in free software.
You could always use a network tool, like iftop, to see network traffic on your PC. That could be a way too see if a program is phoning home. But you'll probably want to use a suite of tools.