Often, after an interview is over, we spend days mulling over what the interviewer may have liked or disliked about our background and how well our skills stacked up against the competition. We remember all the details we forgot to bring up and wonder if that was the decisive factor in our not getting an offer (if we got the job, who cares about the interview?)
At the end of any interview, you are likely to be asked if you have any further questions. So go ahead and ask how you did!
You might try something along the lines of: “I believe that my skills and experience fit the position you have open but I would really appreciate some feedback from you regarding how you see my strengths and weaknesses and if there is some skill or specialized knowledge I need to acquire to be really competitive in this field.”
After the initial surprise (applicants rarely ask such a question), the interviewer will tell you what they liked and also any concerns they may have about some deficiencies. This gives you the opportunity to add some information to overcome those concerns, if you can. Then, if you are not offered the position, you have the chance to figure out how to address those perceived deficiencies before your next interview.