When he wakes up and finds some wetness on his clothes and doesn’t know what it is
Question

I have an awkward question. What do you do when, upon waking up, you are unsure if you need a fardh ghusal? In other words, you are not sure if you had emmitted semen whilst sleeping for whatever reson (the signs aren't visible, or only partially etc.) and you don't know whether a bath has become obligatory upon you. Please advise me on this situation.
Praise be to Allah.When a person wakes up and has had an erotic dream, but he does not find any wetness on his clothes, then he does not have to do ghusl, according to scholarly consensus, because the Prophet (peace and blessings of Allah be upon him) was asked by a woman, “O Messenger of Allah, Allah is not shy of (telling you) the truth. Does a woman have to do ghusl if she has an erotic dream?” The Messenger of Allah (peace and blessings of Allah be upon him) said: “Yes, if she sees water.” (Narrated by al-Bukhaari, 282; Muslim, 313). This indicates that it is not obligatory to do ghusl is one does not see water. Al-Mughni, 1/269. 
But if one finds wetness, this must mean one of three things: 
1 – That one is certain that it is maniy (semen), in which case ghusl is obligatory according to scholarly consensus. Al-Mughni, 1/269.
2 – That one is certain that it is not maniy, in which case ghusl is not obligatory, but this liquid must be washed away, because in this case the ruling is the same as the ruling on urine. Al-Sharh al-Mumti’, 1/280.
3 – If he is not sure what it is, and does not know whether it is maniy (semen) or madhiy (prostatic fluid). In this case there is a difference of opinion among the scholars: 
Al-Nawawi stated in al-Majmoo’ (2/146) that the rulings on both maniy and madhiy apply, so he should do ghusl to remove the janaabah (impurity following sexual activity) on the basis that it may be maniy, and he should cleanse his clothes of the impurity on the basis that it might be madhiy – because he cannot discharge his duty of purifying himself without doing that. 
The madhhab of Imam Ahmad, which was also the view preferred by Shaykh al-Islam Ibn Taymiyah, is that if he was thinking thoughts of desire before he fell asleep, or if he had played with his wife or looked at her, then this wetness should be counted as madhiy, because the liquid that is emitted because of those things is usually madhiy and the basic principle is that it is not anything else. So he should cleanse his clothes of the madhiy by sprinkling them with water, but he does not have to do ghusl. 
But if he did not think thoughts of desire, or play with or look at his wife before falling asleep, then this wetness is to be considered as maniy, because of the report narrated by ‘Aa’ishah who said: “The Messenger of Allah (peace and blessings of Allah be upon him) was asked about a man who finds some wetness and does not remember having an erotic dream. He said: ‘He should do ghusl.’ And he was asked about a man who thought that he had had an erotic dream but he did not find any wetness. He said, ‘He does not have to do ghusl.’” (Narrated by Abu Dawood, 236; classed as hasan by al-Albani in Saheeh Abi Dawood, 216.
 Al-Khattaabi said in Ma’aalim al-Sunan: “The apparent meaning of this hadeeth is that ghusl is obligatory if one sees wetness, even if he is not certain that it is maniy. This opinion was narrated from a group of the Taabi’een, including ‘Ata’, al-Shu’bi and al-Nakha’i.”
 This wetness has to have come out for a reason, and there is no apparent reason other than an erotic dream, and the water that usually comes out because of an erotic dream is maniy. So this uncertainty may be dealt with by going by what is most usual.
 See al-Mughni, 1/270; Sharh al-‘Umdah, 1/353.
 Both opinions are valid, but if you follow the second view it will be sufficient, in sha Allah. If you want to be on the safe side and be sure that your prayer is valid, and you follow the first opinion, that will be better.
 We ask Allah to enable us to understand our religion, and Allah knows best. May Allah send blessings and peace upon our Prophet Muhammad and upon all his family and companions.