If one experiences lust during prayer and places a cloth to avoid distinguishing between seminal fluid (madhy) and other discharges, is what is seen after removing the cloth considered madhy or other discharges?

If you are certain that pre-seminal fluid (madhy) has been discharged, then you must purify yourself from it and perform ablution (wudu). If you are unsure whether the discharge is madhy or vaginal secretions, you may consider it to be vaginal secretions, in which case you do not need to perform istinja' (washing the private parts). However, if you perform istinja' as a precaution, it is better. The mere sensation of lust does not mean that madhy has been discharged. A worshipper should be humble and focused in their prayer, avoiding thoughts of worldly matters and desires.