When there are SE sites that are decided to be closed, are there ways to know it? Thanks!
There won't necessarily be an official hint that a site will be closed, but you can tell by looking at the site itself and observing certain patterns:
Are people voting? If not, that's a red flag. No community can survive if the community itself isn't separating bad content from good. Additionally, a healthy site has high rep users who can vote to close, edit, and perform other tasks related to community moderation. Without voting, there won't be enough high-reputation users to spread community moderation efforts.
This is often overlooked. Is the site generating good, quality content that drives people back to the site? If poor quality questions aren't being edited or closed, then this raises doubts as to how the community will sustain in the long run.
If the site is about extended discussions, polls, list questions, shopping questions, and all of the things that Stack Exchange isn't designed for, this raises concerns about the site's future.
Content Generation and Site Growth:
A small community isn't a bad thing, but it's a concern. If enough core users stop participating, the site could die. Therefore, it's critical that the site show signs of growth, even if it's small.
If there is little interest in the site governance issues, lack of passionate users, and decisions being made only by moderators, then this raises some issues as well. A Stack Exchange site should be full of passionate users who care about the direction of the site.
If you have concerns about a site you belong to, voice your concerns on meta now. Once the Stack Exchange team decides to close down a site, the next step is to start over with a new proposal.