is like ね。, but implies you really aren't so sure anymore, looking for verification.
It should only be used after a copula (だ・です) or a Past tense; explanatory んだ is fine too.
Note that combined with the copula (だ・です), it becomes だっけ, which is different from だけ (= only this thing).
誕生日は三月だっけ。 Your birthday was in march, right? (What was it again?)
レポートの提出は君が遣ってるんだっけ？ as for the submitting of the report, you are doing it right? (What was it again?)
で、どこだっけ eh, where was it again?