Does one have to perform ghusl when in doubt about the liquid discharged, whether it is wadi, maniy, or madhiy?

The discharge that necessitates ghusl is semen only. If there is doubt between semen (manī) and pre-seminal fluid (madhī), the default is that ghusl is obligatory, based on the hadith: "A man finds wetness but does not remember a wet dream?" He (the Prophet) said: "He should perform ghusl." However, if it is more likely to be madhī, then ghusl is not required; rather, he should wash his private part and whatever it touched. As for doubt between manī, madhī, and prostatic fluid (wadī), it does not necessitate ghusl.