is asentence ending particle similar to かな。, by expressing the speaker's uncertainty about the previous statement.
Originally, かしら was an abbreviation of か知らぬ, or かしらん, which are both ways of expressing 'whether it's (A), I don't know'.
it is a very #feminine way of saying.
明日は晴れるかしら。 Will it clear up tomorrow, I wonder
以前お会いしませんでしたかしら Haven't we met before, i wonder
彼は本気かしら I wonder if he's being serious