Can you please tell me how a doctor can tell from an xray if a solid lumbar fusion has occured? does this need to happen before returning to work?

Yes and no: Bony fusion is seen by bone growth and fusion of the bones which can be seen on x-ray. This can be difficult if there is metal hardware present. CT scanning is better than plain x-ray. Again, hardware may get in the way. I am not a surgeon but in my experience, you don't need to see bony fusion in order to return to work. However, you need to consult with your surgeon.