If your community is public, track the number of people arriving by search over the past year. (Google Analytics > Acquisition > All Traffic > Channels > Organic Search).
If this number rises (regardless of what else is happening) you’re probably going to be fine. You have a growing number of people arriving in discussions in your community. That’s a good sign.
If however, the metric is beginning to decline. You have a problem. Fewer people are reaching the community. Your source of newcomers will dry up fast.
This means either your community is ranking lower in search results than it used to (check Google Search Console) or fewer people are searching for relevant terms. If it’s the former, you need to invest in technical SEO.
If it’s the latter, you need to either change the terms you’re optimising for or change the entire concept of the community.
The image above is from a client whose community might just be beginning a downward trend. The longer the decline continues, the harder it is to reverse.
Make sure you respond as quickly as you can.