Answer:
1) At the end of menstruation
2) At the end of nifaas (after birth bleeding)
3) Ejaculation of semen out of excitement. The semen of a woman is yellowish in colour.
4) Entry of the glans of the penis into the vagina (even if ejaculation of semen does not take place)
Ghusl, however, does not become obligatory due to the secretion of mazi (pre-coital fluid) from the private part. Mazi is a thin clear liquid which is discharged at the time of arousal without ejaculation and it can be secreted without one being aware of it. This necessitates wudhu on emission, not ghusl. Mazi is more common in women than men.
ِAlso note that mazi is impure and has to be washed off before performing salat.
Therefore, if due to arousal your wife has a discharge and she is not aware of the discharge until she sees it or feels the wetness, the discharge would most probably be mazi, and therefore ghusl will not become obligatory. If however, the discharge is accompanied by orgasm, then it would be semen, and therefore ghusl would become obligatory.
( Bashishti Zewar 1/69 )
( قوله : لا عند مذي ) أي لا يفرض الغسل عند خروج مذي كظبي بمعجمة ساكنة وياء مخففة على الأفصح ، وفيه الكسر مع التخفيف والتشديد ، وقيل هما لحن ماء رقيق أبيض يخرج عند الشهوة لا بها ، وهو في النساء أغلب ، قيل هو منهن يسمى القذى بمفتوحتين نهر (رد المحتار: 1 ص 335 )
Mufti Faizal Riza