Great question! Presenting virtually IS quite a different experience (and a different skill!) than presenting in person. Some speakers who are GREAT on stage will flop in a virtual setting. After preparing for this transition for several years, I think I can help you spot the ones who aren't ready for the small screen.
1. Start by seeing how that speaker presents in video form. Many speakers have a YouTube channel and deliver their content in direct-to-camera videos.
2. Better - but maybe less likely to be available - is their live stream replays. Granted, a Facebook Live broadcast to their friends and family might not indicate exactly how they would be in a formal presentation, but many speakers and experts HAVE delivered their content in this fashion. Even in casual live streams, you can see how well the speaker engages the audience, uses humor, how professional the quality of their video and audio is, and how well they interact with and incorporate a side chat into their presentation. Do they fumble with the technology or their slides? Do they know how to deal with unexpected situations like background noise or technology glitches? All important factors.
3. Jump on a Zoom call with them and see for yourself. Invite them to give you a 5-10 minute sample presentation to your speaker selection committee.
4. Perhaps most simply...listen to the questions they ask. Do they sound tech savvy? Do they sound overwhelmed? Or do they just say "yes" to the gig without asking any questions at all? (That's a bad sign, by the way...)
The takeaway is this, the best indicator of future performance is past performance. The good news is, their past performance may be somewhere on the internet, available for you to see, right now.