Can a man know know if he has impregnated a lady right after sex?
In other words, ejaculating into a woman and seconds, minutes, or even hours later proclaiming that the woman is pregnant? Not at all. This is something even medical doctors couldn’t tell you right then and there.
At best, the guy will know that he has ejaculated within the woman, void of condom. The woman may have previously told the guy that she was ovulating and not taking any birth control measures. If these things are true, the guy (and woman) would know that there’s a better chance than usual for pregnancy, but that’s about it. The woman would still needs about a week or a couple of weeks before she could get confirmation on the pregnancy.
Until then, we only have a good guess based on facts during the act. @dickhead256