け is a
sentence ending particle in Japanese that is used to convey when the speaker is trying to either remember, or confirm information that they have forgotten. In this way, it is very similar to the 'again' in 'what is (A) again?'
け may only be used after the past form of the
auxiliary verbs だ or た. This means that け (should) only be seen as だっけ or たっけ.
今日（きょう）の朝（あさ）は朝（あさ）ご飯（はん）を食（た）べたっけ。
Did I eat breakfast this morning?
あれ、テイラーの車（くるま）って青（あお）かったっけ。
Um, Taylor, was your car blue, again?
土曜日（どようび）って暇（ひま）だっけ。
Was I free on Saturday, again?
あの人（ひと）誰（だれ）だっけ。やばい、思（おも）い出（だ）せない。
Who was that again? Shoot, I can't remember.
It is also common to see のだ and んだ utilized to make verb tenses a possibility.
クミコって納豆（なっとう）食（た）べれるんだっけ。
Kumiko, were you able to eat natto again?
レポートの修正（しゅうせい）は君（きみ）がやってるんだっけ。
You are the one editing the paper, right?
Despite the grammar rules related to
っけ, the younger generation quite often uses it without the addition of んだ (especially with verbs).
While it is important to understand that this is still
っけ, it is not a grammatically correct form that may be used in writing (or formal speaking).
け will sound equally natural when asking a rhetorical question to yourself, or when directly asking another person about something that you have forgotten.
あれ、カギ掛（か）けたっけ。
Umm, did I lock the door? (Asking oneself)
あそこのレストランって犬（いぬ）を連（つ）れていけるっけ。
Do you remember if we could take our dogs to that restaurant over there? (Asking another)