Is there a way to tell whether よく is an adverb or frequency?

[yangmuye]

已然形 is, I believe, the one of the only six Japanese verbal stem forms that indicates potential/ability.

I'm sorry, 已然形 doesn't exist in modern Japanese.
In classic Japanese, you use “能(よ)く～す” or “～す可(べ)し” ... to express “can do”, “すべからず”,“する能わず”“せられず” “～得(え)ず”to express “can not”.

可能形 is the de-facto name for 書ける 言える 見れる 話せる …(sometimes includes できる 食べられる... depending on the context)
But I don't think it's a good name, do you?
可能形 is usually not used to express 可能性, 可能性 is usually not expressed using 可能形.

[Hektor6766]

It's colloquially called 仮定形, I believe.

[NileCat]

Thanks again, yangmuye.
I found a brief thesis which might be able to reinforce your good explanation.
http://ci.nii.ac.jp/naid/110004625795
My instinct as an native speaker told me that よく作る(make sth well) sounds "wrong". But it seems that's just a matter of semantics, which makes me feel that discerning the "right" meaning of the adverb might be tougher for many Japanese learners than I thought at first...

[yangmuye]

いいえ。こちらこそありがとう。お薦めの論文を拝見しました。とても勉強になりました。