Get an x-ray machine? They are surprisingly cheap pieces of hardware if you are willing to deal with a small area, low penetration image. Low penetration means no lead, which makes for something that's about as cumbersome as a large bar fridge.
It's mentioned in the article that X-Ray didn't help much: 'Gray or off-white in color, they looked more like signal conditioning couplers, another common motherboard component, than microchips, and so they were unlikely to be detectable without specialized equipment'
This comment is specific to the parent talking about their experiences producing credit card terminals that ended up with PCBs implanted in them. Here it is appropriate.
And as soon as they know you are using X-Ray to test ( I would be surprised if there were no insider information leaking on the OP's end ), they would develop a way so that somehow the X-Ray images looks identical. May be a heat spreader like metal as cover up.
