Haven't known it to happen too often but ....
Best detection tool - Mark 1 Nose. If it is burnt/overheated materials then the smell usually lingers in that vicinity.
Any circuit breakers tripped?
If not evident after the event then try to reproduce the conditions it was noticed whilst keeping a good nose and pair of eyes in the suspect vicinity.
Most common cause of "smoke in the cabin" I know of is a faulty air conditioning or engine bleed air system. If any oil gets into the ECS then this can be smelt and in extreme case seen in the cabin air. Causes of this could be a seal failing in the engine, over filling the oil, a cold air unit failing amongst others.
Get a fire in flight - you're in the kack. About the only way to get access behind a panel without tools and the know how is to attack it with the fire axe and discharge a fire extinguisher in there. This in turn can make matters worse because some areas are designed to starve a fire of oxygen and make it self extinguishing (eg lav and galley bins).