Easiest way to check if IFNRg is functional to detect activation of IFg response genes. There whole bunch of them you could find a list. Taqman probes for them a very well established, You could find all information in a INTERFROM database described here:http://nar.oxfordjournals.org/content/37/suppl_1/D852.full . Another way is detection of phosphorylation of downstream factors such as Jaks and STATS, it is very well established method too. If neither Taqman or Ab for Jaks/Stats are available to you than there T-cell morphology/function tests, I am not much know about. Search for those on the web. Good luck.