To quickly find a community you're a member of (unless like me you've joined all 174 of them), navigate to the Accounts tab of your network profile with this link:
Beta sites are listed there just like the others. If you don't know the link, you can get there via your profile page on a site, and then the link 'Network Profile', then the tab 'accounts'. Or via the top bar on https://stackexchange.com/ when you click your avatar.
To find all Stack Exchange communities, please scroll down a bit, you'll find a section called
MORE STACK EXCHANGE COMMUNITIES
with a search bar:
Just type words that appear in the site name or description: