Possible to detect presence of nuclear bomb? Take, for instance, a W-80 nuclear warhead. Does technology exist, say in the form of a satellite or drone, that could detect the warhead's presence (assuming it's not enclosed in some kind of shielding)?
 A: No. It's not even possible to detect one that's inside a suitcase or a shipping container. There's a famous story about how a senator asked Oppenheimer in 1946 whether terrorists could blow up New York this way, or whether there was any tool that could detect the bomb when it was brought into the country. Oppenheimer famously replied, "a screwdriver" -- meaning that you would have to open the box to find out.
Although considerable effort has been dedicated since then to trying to improve detection techniques, highly enriched uranium (HEU) is particularly difficult to detect. It only emits alpha particles, and therefore all you have to do is wrap it in newspaper, and its radiation becomes undetectable.
The APS has a publicly available report on this topic, which goes into some of the physics. There are passive and active methods. Active means that you do something to the material in order to test it, as opposed to just trying to detect the radiation that it's putting out. Re passive detection of HEU:

Currently, passive detection is accomplished primarily by observation of either neutrons and/or photons emitted by spontaneous fission and by photons emitted in radioactive decay and neutron capture. Highly-enriched uranium (HEU) emits a number of relatively intense low-energy gamma rays that are largely absorbed by the material itself and are easily absorbed by most surrounding materials. The more penetrating photons emitted are of low abundance. If the HEU contains reactor-irradiated material, significant contamination by 232U can be found that may be detected through the emission of the 2615-keV gamma ray in the decay of 208Tl. Emission of neutrons from highly enriched uranium (HEU) is quite weak because of the low rate of spontaneous fission.

There are active methods of detection, but they involve radiation that you can't expose people to.
