There's two possibilities and I'm pretty sure one of them hasn't blown their load in me recently.
I changed the code few versions ago so that Sveta will consider much wider range of people she consciously had sex with before she ended up pregnant as potential fathers to avoid the scenario where she has constant sex with a person and ends up pregnant but has no idea how that happened. Now everyone who was inside her vagina before the pregnancy is a valid potential father, condom or no and regardless if she thinks she was on the pill or not.
That all being said, somebody was crafting some option to have a paternity test but as far as I know it's not done yet. So in the game you can't currently find out for sure who the father is. But in the code the data is stored and available if you absolutely must know as a player. If it's your first pregnancy then
$npc_usedname[$ChildFath[0]]
into console should work, if not then replace the zero with the current kid number-1 as the array starts from zero.