To be certain:
1. Consult your owner's manual: This is the best place to start. It should have diagrams showing the location of all major engine components.
2. Use a repair manual: A Haynes or Chilton manual for your specific year and engine will have detailed diagrams and instructions.
3. Online resources: Search for "[your engine size] Ford E-350 EGR valve location" on YouTube or a car repair forum. Many videos show the location visually.
Keep in mind that accessing the EGR valve often requires removing some parts for better visibility. Be prepared to work in a somewhat tight space. If you're not comfortable doing this yourself, take it to a mechanic.