How does the seeker of guidance know that a matter holds no good, so he turns away from it? And is it permissible for a woman to supplicate and give charity to win back her fiancé after he broke off the engagement?

Istikhara is prescribed when one is contemplating a matter, such as travel, trade, or marriage. It is sunnah for the person performing Istikhara to proceed with the matter and take all measures to ensure its success, then submit to Allah's decree. It is permissible to pray for a specific person to be one's spouse and to supplicate by means of righteous deeds for that purpose. It is advised to be content with Allah's decree and to ask Him to facilitate what is good.