This is always a tough situation, when the patient sees swelling but the doctor doesn’t. In this situation, as you mention, ultrasound or MRI can be used to determine whether there is inflammation in the joints or not. If MRI cannot be done, then the radiology facility has to have an ultrasonographer who is experienced with the joints. THis is not as common as one would expect. But, that is a good next step in the situation described here.
Diffuse pain can also be due to non-inflammatory conditions like fibromyalgia.
Good luck !!