Answered on Reddit by a dude named Omicron-Persei-VIII, to wit:
“The actual explosion would be quite similar from a distance. There be a lot more gamma rays that may be detectable from a distance if you are looking for them. However, you’d be able to tell quite easily from the aftermath as it would be a much “cleaner” explosion.
With fission, you’re splitting heavy nuclei like uranium or plutonium. Assuming it even all splits, there’s going to be some uranium and plutonium simply blown away. You have the daughter nuclei still left over from the split. The daughter atoms are all also unstable, they are far too neutron rich. As you move up the periodic table, you need more and more neutrons for an atom to be stable. Hydrogen takes none. Light ones like helium or carbon take an equal number of protons to neutrons. By the time you get to uranium 235, you need 143 neutrons to 92 protons to simply get an unstable nucleus with a long lived half-life. Splitting uranium means you still have a lot of neutrons, far more than the daughter elements are stable with. In addition, some lone neutrons fly off. These can hit other uranium atoms and cause them to undergo fission (hence the chain reaction), but they can also strike other atoms such as the bomb casing making them into radioactive unstable elements.
So you now have a lot of new radioactive elements. Some decay almost immediately, but their decay products are also usually radioactive. The chain can go on for some time until it hits a stable isotope. Most are rare, because they are radioactive with relatively short half-lives. These are isotopes you don’t see naturally like Strontium 90 or hydrogen 3 (aka tritium). Or even elements you don’t see naturally like Technetium. A bunch of very obvious signs the explosion was a fission explosion.
Fusion is different, in this case you are combining light elements to make heavier ones. We don’t fuse hydrogen like the sun though, that’s far to hard and slow of process. We fuse deuterium and tritium (hydrogen 2/3). The resulting product is stable helium, but it emits spare neutron radiation. Which as mentioned previously, can strike other heavier atoms and make them unstable. As well, every fusion weapon (also known as thermonuclear or hydrogen bomb) is also a fission weapon as well. Fission drives the fusion reaction, and then the neutron radiation from the fusion drives more fission.
Anti-hydrogen (or any higher element) annihilation makes neutrinos and gamma rays, eventually. Some exotic matter will be made first like muons (heavy electrons) or pions (things made of quarks that’s aren’t proton or neutrons), but extremely short lived. Even if not short lived, like say the positrons themsleves initially there of from the annihilation of the quarks, if it’s blow away it will react with the first regular matter it hits so it’s not getting far. Neutrinos fly through the earth and off into space with almost no interaction. Gamma rays will cause the immense pressure and temperature of the explosion. The gamma rays and explosion may be so powerful it would cause some radioactive products to be formed. However, the amount would be much lower than the fissible material and neutron radiation from a nuclear weapon. It also wouldn’t be the telltale products expected from uranium and plutonium, but other radioactive isotopes.
Tl;dr: Nuclear weapons, be they fusion or fission, make radioactive fallout. You’ll notice that. Theoretical antimatter weapons would not. You’ll notice the lack of expected fallout.
That’s not to say anitmatter bombs would be some improvement on nuclear weapons. Still outrageously dangerous. Plus, they’re fail horribly wrong rather than fail safe like nuclear. It’s hard to make a nuclear bomb go boom. It’s even harder to make sure anitmatter does not go boom.”
There you have it.