How did he know he had Pre-Ejaculated, because the Pre-Ejaculate is what it says - 'before (pre) ejaculation'. The P-E is not ejaculated. So P-E is only known when seen or felt (wet clothing or on hands by touching the P-E).

If his penis never came out of his boxers (you are sure about this?) the chances are that any sperm would not have got onto you. If he touched his penis or the wet boxers, he could have got some on his hands. Putting his fingers in your vagina could then have made the transfer. Likewise you could have done so if you got any on your hands and you masturbated.

Does this help?