There's no way to tell when conception happened. That's why doctors date a pregnancy using your last period. Due dates can be off by a week or two in some cases.
Also, you don't conceive that exact night you have sex. It happens after. You need to be ovulating for conception to happen. Sperm can also live inside you for up to 5 or so days too so you could have had sex a good few days before you actually ovulated and eventually conceived.
Unless this is a paternity issue then it doesn't matter when conception happened, and there's no way to know now.