I do know a lot about pussy stains in thongs, after all i lived with kiky for about ten years and one of the first things i used to do every morning was licking and sniffing her underwear from the previous day.
in general, for me, pussy stains only indicate the health state of a lady, the smell and taste indicates weter her pussy is healthy or not. it she´s clean and not to slutty the smell and taste must be softer and sweeter, if not it would be stronger and sour.
but if you know a girl for long enough time you can also see the changes during the month, the stain is bigger, clearer and sweeter in taste and smell when they´re close to/or ovulating. also the size of the stain could indicate that she got turned on by someone or something.
I remember that everytime kiky blowed me her thongs would not just have pussy stains but would be stiff because of all the dryed pussy juices, so when she started to give blowjobs in exchange for promotions, to close bussines deals and etc. I always knew she had done it, even the times she forgot to ask for my authorization or to tell me after she did.
and of course if the girl got fucked wearing the thong the stain is huge, and it will always smell to cock and if she was creampied the smell of cum would be undeniable. Kiky nevr had been a huge fan of creampies or cum on her pussy, specialy because she can not take pills, but every now and then she would get home with a cummy thong and the stain is very different.