Appliance Inspection for Home Inspectors](http://www.nachi.org/appliance-inspection-course.htm)… free, online course.
InterNACHI’s Inspection Checklist and Precautions for Testing Garbage Disposals:
“If the home has a double sink, check to make sure the waste pipe from the disposal has a trap installed.”
The waste line can join the drain at the opposite sink before the trap.