Hey Nikita,
I use UI Browser for such things, but at $55.00 U.S. it's a bit expensive for most non-professional scripters. I bought it over a decade ago after discovering how convoluted GUI-Scripting was, and I've never regretted spending the money.
Xcode comes with the Accessibility Inspector utility, which is useful but not nearly as helpful as UI Browser.
There are other fairly brute-force discovery methods as well.
References For Learning & Using Applescript
Learning & Using AppleScript & JavaScript for Automation (JXA)
-Chris