Search the IDE (folder ide): Any code that is not in "ifdef windows" or "ifdef MacOs" runs on both systems. That is a lot of code.
However: You should always test on both, if you want to ship both. Even if a feature exists on both, it may have a bug on one or the other. Then until the bug is fixed, you need to use something else.
And some features are platform specific: On GTK, you can write an app with close buttons on each tab of a pagetontrol. It will compile, and run on Windows. But the close buttons are not there, and event for them will never fire.