Unfortunately there really is no way to tell who the father is until you have a paternity test done. There is no point in worrying yourself now, especially when you are pregnant. You need to try to stay as serene as possible for the health and well being of your baby. The chances of the first guy being the father in my opinion are definiately higher than the second guy for obvious reasons but there are no guarantees.
Do you have a support system? Friends or family, maybe a mom's group? It's very important to have help and support when you are pregnant...especially when it wasn't a planned pregnancy.
I wish you luck and hope the outcome is as you want it to be!