How could the doctor tell if my back pains are chronic or not?

It's all about time. The label of "chronic" or "acute" when referring to back pain will depend on the amount of time one has had symptoms. Generally, acute pain is pain that is less than 2 weeks old. Chronic is when pain as been present for longer than 6 weeks. The length of time one has pain will play a role in how the doctor treats the pain.
Time. The length of time youvhave had the pain.The word chronic describesvthe past dosent predict the future cut offs vary but 3-6months have been used.
Pt tells Dr. In simple terms...Chronic means "old", acute means "recent". The patient is who tells the dr. For how long he/she has had the pain or symptoms. Said that you may be asking how can i know if the lesion I have in my back is recent or old? The radiologist have some criteria to read images like "swelling in the lesion ( acute) and callous formation (old)in a bone to determine the age of a bony lesion.