Is it obligatory to perform ghusl upon waking up when there are water stains on the underwear, and it is not known whether they are semen or not, and they do not have the smell or texture of semen, given that this happens repeatedly? And is it permissible to differentiate between semen and other substances by smell and texture?

Whatever you find on your garment after sleep, if it does not have the smell or color of maniy (sperm/seminal fluid), then it is not maniy and does not necessitate ghusl (ritual bath). One can differentiate maniy from other fluids by its smell, color, or one of these three signs: its emission with desire followed by a subsiding of that desire, or its smell which resembles palm spathe or dough, or its emission in spurts and gushes. Any one of these signs is sufficient to consider it maniy. If you are uncertain about the nature of the fluid, you can either consider it maniy, perform ghusl, and repeat the prayers, or consider it madhiy (pre-seminal fluid), in which case your prayer is valid after washing the impurity and performing wudu (ablution). If you prayed with madhiy impurity out of ignorance or forgetfulness, you do not need to repeat the prayer. As for being awake, ghusl is not required for maniy if it is emitted without desire.