Firstly, connect a vacuum pipe to the petcock, block the fuel outlet and suck on the vacuum pipe - this will show whether the petcock is leaking.

If not, connect a bottle to the fuel line (or, put the fuel tank on a shelf and run an extension hose to the fuel line) to run some fuel in and that will show where the leak is. (A quick and dirty would be to blow gently into the fuel line and feel to see where the air is escaping but …).

 

From what I can tell from the last picture, that appears to be one of the fuel T's that distribute fuel between two carbs. If that is the case, they are known to crack or the o-ring on wither end of the T (carb side) could be bad. Given the amount of fuel in the video, I'd guess it would be a split rather than a bad o-ring.

 

Just to let you know, you can just prop the tank up and run the bike to get a better view of what's happening. There should be a prop mechanism to do this.