If your LO fequency is F_LO and your desired signal at F_RF, your downmixed signal will be at F_IF = |F_RF-F_LO|. The image frequency which, undesiredly, will end up at the same IF frequency will be at F_M=F_LO- (F_RF-F_LO). The most straightforward way to measure the image suppression is to inject F_RF with a certain power level P and measure the power at F_IF, then inject F_M with the same power level P, measure the power at F_IF again and divide the two results.