If someone doubt that he/she is the situation of jannabat. For example he/she dont know exactly what is to be in this situation. What does he/she has to do?
Is it obligatory to perform gusl?

Also, how a single women can know if she is najiss or no? I mean, if we see a fluid in the underwear, then we can consider that she is najiss
But if she see nothing , or just feel a fluid inside , but it does not spread out, she still najiss? Or no need to do gusl?

The fluid which a woman discharges in the state of lustfulness and which is called ejaculation – which normally takes place during orgasm – is treated as semen and it will cause janabat (in which case ghusl becomes obligatory). That is to say, ghusl becomes obligatory irrespective of whether there is climax or not. As for the fluid which comes out without lust or the fluid that comes but does not spread out to the underwear and which is only created with little sexual provocation, it does not cause any taklif (obligation) and if there is little fluid that reaches even the underwear but it is little, it is not considered to be ejaculation, so ghusl does not become obligatory.