Hey Martin,
Basically you can't do this on macOS – i.e. it has no general API to allow a utility to test whether an edit environment is in use at a given moment.
There are some workarounds that can work in certain circumstances, but there is no panacea.
Search the forum for "text field", and you'll find some discussion. Here's one example:
Can I check if I am in a text field?
-Chris