Is ghusl required upon waking from sleep and finding a very small amount of fluid, no larger than a lentil, inside the urethral opening, when it's impossible to distinguish whether it's semen or madhiy, especially while suffering from waswas?

The questioner is not required to perform this type of examination, as it is a form of waswasa (obsessive whispering). What he finds has not exited to the outside. The jurists have explicitly stated that what exits from a man's penis is that which reaches outside the glans. What the questioner finds confirms that he is not obligated to perform ghusl (ritual bath), even if he were to confirm that something exited but did not know whether it was mani (sperm) or madhi (pre-ejaculatory fluid), he would still not be obligated to perform ghusl.