BTW, shipping is way down. Many countries have paused shipping to the USA. Even container booking are at an all time low. So they are at an all time low on inspections. After midnight though, there may be nothing clearing customs. They may all go on vacation. When they do start back up, it will take weeks and weeks to cover the backlog.
They actually do train the fellows doing this. They do have access to the declarations, it is attached to the package. They are perfectly aware what a drone is and how to identify it. They are supposedly blocking them from import, so I don't think they are using psychics. The new scanners are actually quite detailed, show different densities in color. They are a bit more like CT scanners, not the x-rays of old. Batteries are usually flagged. There is a check to see if they went on a conforming flight. There is a sizable fine for misrepresenting the contents and if it has batteries. I had to take lots of boring hazmat cargo flight courses. Lithium batteries are restricted to shipping on certain flights. So they do check, especially since they are now part of the stacked tariff codes. Heck, we even are required to carry a box with UN hazmat labels of all kinds to replace damaged ones if we get one that peels off or is damaged. They are very easy to spot. Do you not travel? stand and look at what is in your bag at baggage inspection. They don't even have the really good scanners.
I always treat anything with customs with kid gloves. I have fired pilots, on the spot, for pissing off customs officials. The officials have quite a bit of power and there are very few rights crossing borders. An unaccompanied package has no rights. If they are suspicious of a package, they can take and blow it up if they think it may be dangerous. No need for a lot of proof. They are aware people are trying to find ways around the tariffs. There is a whole task force set up to identify the paths of third country export. You really need read up a bit more on what is going on. Kind of a good time to sit on your hands about this stuff.
There is one glaring fail on all these scanners and inspectors. None seem to be able to detect drugs. So if you really want to get a drone in from overseas, have the shipper carefully pack it in cocaine. Renders everything inside invisible.