As an individual?

• Browse for communities to sub on lemmyverse.net rather than your instance's community list. It will show apples to apples activity rankings.
• If you do use your instance's community browser, cross-reference subscriber counts by visiting the community's page on its home instance. That will provide a comparable sub count to what's shown for local communities on your instance.
• Pay attention to how mods behave. Do they have a well-specified set of rules in the sidebar? Do they organize megathreads when necessary? Stalk them on the modlog to see if they are fairly enforcing the rules or power-tripping. This gives you a metric other than size to evaluate community health. One prominent sports community on Lemmy.world recently became the largest for its topic in the most recent user-signup wave while effectively being unmoderated. It overtook an older, more established, and better run community on another instance and it's too late for accurate size-metrics to help anyone understand the difference between these communities now. You have to look at mod activity to see that one mod team solicited community feedback on rules, enforced them fairly, set up raceday discussion threads, and had fruitful policy discussions and calm disagreements with subscribers. The other was absent for days at a time, ignored mentions, didn't set up rules, ignored conflicts between users, the showed up to remove posts trying to jumpstart policy discussions, and basically was AWOL until some users of the community petitioned the admins to take over the community. It's probably going to head in a better direction now that there are finally more mods on board, but its dominant growth phase occured while an absent mod was squatting it and doing nothing much of use, the only thing that mattered was being on lemmy.world with misleading subscriber counts and that was enough to become what is now genuinely the biggest community for its topic.

As the threadiverse overall, I think community discovery within Lemmy just has to be a lot better. I'm not sure what that looks like in its entirety, but I'm confident that a critical piece of the experience is comparable activity metrics for local and remote communities being prominent in Lemmy's native community browser.