To judge a technical ability of programmer ask about his past technical work, ask some general tech questions and If you can, try to give him/her a little challenge where he/she can prove some of the skills he/she have enumerated on his/her CV and that he/she will need for the job. In a further interview you may ask him/her how he/she solved some problems. If it is the case, ask him/her how he/she did certain aspects of items listed in his/her portfolio.