Back pain jenuine not faking

Patient: Dear Dr, I have very bad back pain and my GP said pain is subjective but we are not sure where the pain is coming from. I am 32 year old with grade 1 spondylolisthesis. I just want to know what is the right method to find where is this pain coming from? (jenuine pain not faking)? Is there any brain test method? Thanks and Regards. Harry Kapadiya