Practically there is only one way to confirm regarding child that it is due to you or her husband.you can go for DNA test of yourself and amniotic fluid
of your girl friend .
Other possible thing could be she might not be pregnant and pretending to be ,if so you will be able to realise very soon.One more thing here is to be noted is that UPT cannot come +ve within 14 days of relations so if she is not lying that she is pregnant then she might be pregnant not due to you ,but due to her husband.
I have tried to show you every possible way ,
Hope this clears your doubt and are able to free yourself.
Try avoiding such a situation in future .