I'm inclined to believe they don't.
If it's labeled, they are generally right on.
I recently had a shipment, powder seized. As normal, you get an empty package, with a letter in it and some green tape on your package.
About a week later, i received my official letter. It was powdered form test I had ordered, and in the letter they say it's HGH.
I think they just look at it, assume, and seize cause I doubt my supplier is shipping me HGH at the same price as he would Prop.
In their letters, they are very passive too. I think they just take anything that is suspicious, send out the letter. If they are wrong, you call, they continue to ship through.
If they are right, you don't call, you don't get your package.
I'm not sure how difficult the test are, and how much they would cost, but would be interested to see their response if you called and asked why they nabbed your Chinese powder eggs and request a lab report.