Fluids

Question: What about if the discharge is composed of residual semen which was out of reach of washing and only later is discharged? What about if you feel you cannot be sure which it is?

This would require normally just invalidate wudu, unless the ghusl was performed immediately after the emission of semen or female sexual fluids. If one urinates, walks around or sleeps before ghusl, this is a sufficient interval. 

Wassalam,
Faraz.