Translations:F.A.Q. - Frequently Asked Questions/35/en

Generally you can check the instance rules to see if the instance is well moderated, but some instances may have good rules, abut the admins and mods don't enforce them well.
One indicator for good moderation is a long blocklist of instances. You can see that under domain.tld/more.
You can see a list of instances that you definitely shouldn't join under Template:Internal Link. These should be blocked by most instances.