Sheikh Muhammad Salih Al-Munajjid, a prominent Saudi scholar, stated,
“Madhiy is a sticky white fluid that is emitted when a person is thinking of or desiring sexual intercourse. Its emission is not accompanied by feelings of pleasure. It does not gush out in spurts and it is not followed by exhaustion. This may happen to men and women alike, although it is more common among women, as stated by Imam An-Nawawi in his commentary on Muslim.
Madhiy means that a person only needs to do wudu’. The evidence for this is the report of ‘Ali ibn Abu Talib (may Allah be pleased with him) who said: “I was a man who used to have a lot of urethral discharge (madhiy), so I told Miqdad to ask the Messenger of Allah (peace and blessings be upon him) about it. So he asked him, and he said: ‘It requires wudu’.’” (Al-Bukhari and Muslim) Ibn Qudamah said in his book Al-Mughni, “Ibn Al-Mundhir said: ‘Scholars agree that passing stools or urine, emission of madhiy or passing wind are all invalidate taharah (purity) [i.e. they mean that wudu’ must be renewed]’.”
It is noted that Madhiy (urethral discharge) is najis (impure), according to the hadith of ‘Ali quoted above; in some versions of which it is reported that the Prophet (peace and blessings be upon him) commanded him to wash his penis and testicles and do wudu’. So madhiy is najis and the penis and testicles must be washed, as it nullifies taharah.
To purify oneself of madhiy, it is sufficient to sprinkle water on it, because of the difficulty of doing otherwise. The evidence for this is the hadith narrated by Abu Dawud in his Sunan from Sahl ibn Hanif, who said: “I used to have a lot of urethral discharge, and I used to do ghusl a lot. I asked the Messenger of Allah (peace and blessings be upon him) about it and he said: ‘It is sufficient for you to do wudu’ (to purify yourself) from that.’ I said, ‘O Messenger of Allah, what about when it gets on my clothes?’ He said: ‘It is sufficient for you to take a handful of water and sprinkle it on your clothes wherever you think (the madhiy) is.’” (Reported by Al-Tirmidhi)”
As stated in this fatwa, you may know about your dropping madhi by looking at your underwear in case you feel emitting or in case of strong sexual urge.
May Allah guide all of us to what pleases Him!