You may ask her when was the date she first had her last period and how many days her cycle is.( A female cycle could be from 24- 30 days, with a lot of variations)
If her cycle is regular and lasts e.g. 28 days, she is fertile any time from say the 12th day to the 16th to be on the safe side. ( The count starts on the first day of her last period) Say she had her period on the 30th of April, she will most probably be fertile from the 10th of May to the 15th May. However, these fertility calculations are very basic and are very much influenced by a number of other factors. So, if she doesnt know about her cycle or when her last period was, the only thing to do is wait for a couple of weeks and ask her to have a pregnancy test.
Please forgive me for mentioning this but next time, do yourself a favor and use a condom even if you are intoxicated, to avoid getting something really nasty like Aids on top of getting your partner pregnant.