You'll know when human-to-human transmission starts the same way we knew at the end of 2019 when a "novel respiratory virus" started spreading: random comments on Hacker News.

Then 3 months later the media will start commenting on it.

And 3 months after that the CDC or whoever will acknowledge that it is not a silly fear/hoax.

Look to the level of preparedness that happened for the prior pandemic.

The level of preparedness at CDC/NIAID now is likely about as /good/ as their level was then.

I.e., they will likely be caught out flatfooted yet again, and bungle their way through, just like they did the last time, having learned little to nothing from the process last time.