We at ReFlar have, as Frink mentioned, been updating our extensions for beta 8 compatibility.
We have also been helping Flagrow update theirs, as they are busy with something that will be great once beta 8 comes ?.
Now, @Sanguine the best way to check for compatibility would be, as @Zeokat mentioned, looking at the version requirement in the extension's composer.json.
However, that won't be foolproof, as some may forget to update their composer.json before publishing the version to Packagist.
I guess we'll see once beta 8 comes out.
