A WB would tell you silencing efficacy. For the transfection efficiency per se, your best bet would be RTPCR targeting the siRNA. You should be able to use stemloop reverse transcription strategy as in miRNA quantitation. In situ hybridisation might be another option, depending on your aims.
I agree with both Tarik and Rajaei from above, WB would be more of the gold standard to prove the transfection worked. But if you want a number, qPCR would output numbers better, although using imaging tool, you can can relatively number off of WB result too.