Check by open the battery door.
Clean the Battery Door's Metal Contact Leads with a Pen Ink Eraser or a Emery Sandpaper.
Then look inside of the Battery Chamber Assy, you will see Four Metal Contacts. Use a flashlight to shine and you will be able to see. Clean those Contacts.
If the Metal Contacts, from the door, are deformed, just pry them out a little.
Unless, you are talking about the On/Off Switch. Meaning you are able to remove the flash.
If that is the case, then you have to use the DVM to check in Ohm Scales.