Thank you for your question.
In the Reliance of the Traveller, the author, may Allah reward him, says,
“e 10.6 If one does not know whether one’s discharge is sperm or whether it is madhi (def: (1) above), then one may either:
(1) consider its sperm, and perform the purificatory bath (0: in which case washing the portions of clothes and so forth affected with it is not obligatory, as it is legally considered a pure substance);
(2) or consider it madhy, and wash the affected portions of the body and clothes (N: which is obligatory, as it is legally considered filth), and perform ablution, though not the purificatory bath.
The best course in such cases of uncertainty is to do all of the above (0: of bathing, washing the affected portions, and ablution, so as to take due precaution in one’s worship).“
It also mentions in the Reliance of the Traveller that when semen is dry, it smells like egg-white, perhaps you can smell it and see if that helps you determine the source. If you have decided that it is not mani, it is valid, but do try to actively determine what it might be and then stick to your decision.
I hope this helps, may Allah reward you for your efforts and sincere intention to do the right thing.
[Ustadha] Shazia Ahmad
Checked and Approved by Shaykh Faraz Rabbani
Ustadha Shazia Ahmad lived in Damascus, Syria for two years where she studied aqidah, fiqh, tajweed, tafseer, and Arabic. She then attended the University of Texas at Austin, where she completed her Masters in Arabic. Afterward, she moved to Amman, Jordan where she studied fiqh, Arabic, and other sciences. She recently moved back to Mississauga, Canada, where she lives with her family.