How can semen be distinguished from other vaginal discharges, especially upon waking from sleep, and what should be done when in doubt about the emission of semen so as not to miss the prayer?

"One is warned against obsessive doubts (waswasah) and succumbing to them. Ghusl (ritual bath) is not obligatory unless the woman is absolutely certain, with definitive conviction, that semen necessitating ghusl has exited. As for doubt, she has a choice between the possibilities of what she doubts, and she may assign to it the ruling she desires."