What should I do if I find spots on my clothes and I don't know if they are madhy (prostatic fluid) or wady (a thick, white discharge that often follows urination), and they are difficult to remove? And what should I do if I suspect the presence of something during prayer and then find it after prayer?

"Madhi" (pre-seminal fluid) and "Wadi" (thick white discharge) are impure (najis). Sprinkling water over them is not sufficient; rather, they must be washed with water until their trace is completely removed. Doubt about the emission of anything during prayer does not harm or invalidate it, unless one is certain of an emission. Anything discovered after the prayer is considered to have occurred after it.