What is the ruling regarding the fluid that is discharged after long conversations with the fiancé, and how can one differentiate between mani, madhi, and wadi?

You must fear Allah and abandon conversing with the foreign young man as long as the marriage contract has not been concluded. To differentiate between madhy (pre-seminal fluid) and maniy (seminal fluid), you can refer to relevant fatwas. If you are unable to identify the fluid, you may consider it maniy and perform ghusl (ritual bath), or madhy and perform istinja' (washing the private parts) only.