Injecting the sacroiliac joint (SIJ) with a numbing agent similar to novacain may reduce some or even all of your pain. The injection is both diagnostic and a treatment at the same time. If other words, your pain is better and now you know for sure the SIJ is the problem.
If the injection doesn't help, then your doctor knows to look somewhere else for the source of the pain. It's a win-win situation whether or not it works.