RiversideRocks easiest solution is to look up the extension on Extiverse. There's a badge that will says which versions of Flarum an extension will work with.
If the extension has a discussion on this forum, we should have added the "Incompatible" tag to the discussion.
You can also look up the requirements for flarum/core under each extension on Packagist website.
Or finally if you try to install the extension with composer require, it'll fail telling you it needs a specific Flarum version that will probably contain beta- in the name. You could also use the --dry-run flag to make Composer try to install it virtually without actually installing it for real, that way you see whether it would install without actually installing it if it does.
