Originally,
かしら was an abbreviation of か知（し）らぬ, or かしらん, which are both ways of expressing ‘whether it’s (A), I don’t know’. These days,
かしら is a standalone 終助詞（しゅうじょし） (sentence-ending particle) that works in a similar way to
かも, by expressing the speaker’s uncertainty about the previous statement.
かしら may be attached to the end of almost any phrase that the speaker wants to express doubt about.
Although the original forms of か知（し）らぬ, or かしらん were useable by anybody, かしら is thought to be very feminine in modern Japanese, and therefore is not often used by men.