Depending on how long ago the hashtag was created, you might be able
to determine this with a Twitter search. But the easiest way is
probably to go to http://twapperkeeper.com and create an archive. If
there's already an archive for the hashtag, you're done, assuming the
archive was created before the first tweet disappeared from Twitter's
cache. Otherwise, create a new one and wait for the Twapperkeeper
historical search to search back in time.