I was intimate with my wife and some but not all of the characteristics that semen had come out were present to suggest I needed to do ghusl. I did not do ghusl and fasted the next day. After further investigation on the day I was fasting, I noticed there was a white stain on my clothes. I am not sure if it was semen, and I am in doubt if I was tahir. Would I need to do ghusl and break my fast?
If there was penetration, then both parties become in the state of janabah irrespective of whether there was ejaculation or not. If there was no penetration, and you were in doubt whether you ejaculated or not, then you would not be in the state of janabah, hence your fast is in order.