* Custemers are not signing up and not paying. The most obvious one. But this can be hard to find. But say if you are used to add features or fix bugs customers find, and then all of the sudden nothing. It could mean product is in good shape, or nobody is using it.
* Senior people leaving. Especially developers whose judgement you trust.
* Drastic changes to the product. "Wait, why are we pivoting?" Corrolary: quite often the pivot fails. It is always in the news if it succeeds. but that is only because it is exceptional.
* No bonuses, and salaries have not been going up. If you somehow find out that nobody's salary went up and nobody got a bonus (this is hard often, companies don't want you to discuss those things).
* Maybe a sharp increase in team-building activities. Taking everyone for lazer tag is cheaper than increasing salaries. But it presents this image of "everything is fine". I have seen that -- a few senior people left. All of the sudden a sudden surge of minigolf, bbq, ski trips and other to mask away the issues. But then again, this can be a sign that the team is doing so well and are being rewarded. I guess this is more of a gut feel and depends on the large context (so it is a secondary sign).
* Owners start avoiding meetings and questions. Because they know they might have to lie. It is up to you, but you can try to ask directly. Then you sort of force their hand. That could backfire. You are now "not a team player" and not a "culture fit" so beware.