The best practices I have seen in Blackboard Collaborate for verifying users on video during a presentation is to either use smaller breakout groups that are sized to allow you to see all the videos or do a periodic roll call to bump each attendee up to the featured speaker spot in follow the speaker mode. 

I will pass a note along to the Collaborate team at Blackboard about this use case. Maybe they can consider adding a selector function to the videos, choose who to see, and an indicator for video and audio instead of just audio. It might not be a bad idea to contact support with the product suggestion as well. Blackboard Collaborate might have a more clever workaround.

Zoom does have a bar where you can scroll through all the video inputs from each attendee to verify that they are indeed on. One word of caution, or perhaps feedback for the regulators, all those video feeds use up increasingly more and more of the bandwidth for all the attendees which will degrade the presentation audio and share/video that they are all trying to watch. This can also significantly impact students with low bandwidth connections and who might have to pay for additional bandwidth consumption. The design team for Collaborate deliberately chose to not put all the videos on the screen; to try to save the bandwidth for the current speaker and anything they might be presenting. You can test this out by having a group of people (~10) join a meeting, all turn on cameras at once and then see the quality degrade and then turn them all off and see it shoot back up. 

A best practice for video presentations is to do a roll call and intro with videos on and turn them off during the presentation. They can be distracting and eat up the bandwidth. How do you know if people stayed glued to the screen during the presentation? Pop a poll up periodically and track who fails to respond and then open a test in the course right after the session and see who paid attention (not just left the video on).