That's not how to check if you have transparent background. The best way is to drop it on a timeline over something else.As far as how I see if it exported with alpha, I can tell as it's rendering. I can see some of the computer screen underneath the video I'm rendering.
And I also checked by playing the QT after rendering and the background was solid black.
When I export my clips with prores alpha, I don't see the computer screen underneath and background is black when played with QT. Yet it still has transparent background.