Help needed - Get valid camera indexes

I am looking for a way to get valid camera indexes for usage with `VideoCapture`. I am using Raspberry Pi 4 with 2 USB webcams, currently I can open them using `new cv.VideoCapture(0)` and `new cv.VideoCapture(2)`.

There's a gap between the indexes and if I try `new cv.VideoCapture(1)` I get an error, so iterating through indexes until an error is thrown may not be the best approach.

Is there a method of determining which indexes are valid?

*OpenCV* version 3.4.6
*OS*: Raspberry Pi OS