One of the most obvious indicator of the groups' popularity is when they receive tons of attention during their songs drop or cb whether it's positive or negative. One of the obvious signs when one group started to lose gp and kpop fans attention is when people didn't care anymore about them and talk even less about them even when their songs dropped or during their cb. That's the most scary reaction among all reaction out there for kpop groups.
Kep1er is the perfect example for this. Wadada give them lots of attention even with a huge hate train following them. This forum even pay fully attention on them during that time. But gradually, people just stop talking and didn't care much anymore. So for me, as long as people talk about that one group even to critise them, it's still a good sign for the group to grow.
When people didn't care anymore, the group can released the best song ever exist in kpop but still unable to capture the mass audience attention back.