Ideally you'd get to talk to people who used to work there, though it's hard to see how that would work.

I know back in olden times I'd always look at the "Help Wanted" ads, and would note that certain companies appeared to have constant employee turnover. I took that as a bad sign.

Probably not as valuable a metric if the company is growing rapidly. Also people tend to job hop a lot more nowadays.

I totally agree. Their perspective and thoughts also matter. This kinda goes along with the whole "Do they allow their entire team in interviews".