How to differentiate between waswas and our own thoughts?

Recently I have been suffering from waswas ( whispers ). Am having some disturbing thoughts. However usually I am unable to tell if they come from my own brain or from waswas. For example , This morning there was a small patch ( it was barely identifiable and it had dried very well so could barely see it ) on my underpants which I assume to be either water that I used for istinja the previous night or wadhi or madhi ) . It was unlikely that it was mani because the quantity was somewhat small, it didn't have a strong smell ( I don't know what mani smells like ) beside other signs ( my private part wasn't feeling sticky or anthing didn't seem to have come out andate dried on my private part or I could still get an erection - if you release mani you can't get an erecting for some time ) . In the morning I quickly dismissed that the patch was water or something else, probably not mani. However by midday the thought of the small patch I had seen began to haunt me and during my prayers I wasn't concentrating much and I was always thinking like what if I am in Janabah ? All my prayers are invalid :( I have this internal feeling that I am actually pure but when I think of prayer or other worship I feel impure though that feels like a wrong thought but I can't dismiss it. I initially assumed that to be waswas so I ignored it and prayed without ghusl but now it's late in the day and I feel like it must have been my own thoughts and my brain was putting up a logical argument although i doubt this too. How to identify waswas and our own thoughts separately ? So that we can act upon each accordingly . And are my prayers for the day invalid ? Jazakallhu Khair