Some communities have a list of FAQs in their wiki; to see if your favorite community has a wiki page, look for for a "wiki" link in the right-side column or going to reddit.com/r/name-of-subreddit/wiki directly. You can also find subreddit rules and other helpful information in the various widgets in the right-side column of a community page if you're on desktop. On the mobile app, look for the "About" tab on the subreddit page.