What is the ruling on doubting the nature of dry spots on underwear: are they urine, madhiy (pre-seminal fluid), or wadiy (thick white discharge after urination)?

If you are in doubt between mani (seminal fluid) and madhi (pre-seminal fluid), choose one of them. This is the relied-upon opinion among the Shafi‘is and is the fatwa (religious edict) given by us. However, if you are in doubt between urine, madhi, and wadi (a thick white fluid that is discharged after urination), some Malikis prefer washing the entire penis if one of the three is madhi, acting on the side of caution. Both wadi and urine are impure and must be purified.