As noted, the electron/positron reaction will have a very distinct gamma ray signature at 511 KEV, which is enough for any observer to immediately identify this as an antimatter reaction.
Protons and antiprotons release a violent stew of charged and uncharged particles. Most of these won't be detected directly, since their lifetimes are measured in milliseconds, but they too decay into energetic photons.
Here are the specifics of the reaction that produces the mesons. Protons consist of two up quarks and one down quark, while antiprotons consist of two anti-ups and an anti-down. Similarly, neutrons consist of two down quarks and an up quark, while antineutrons consist of two anti-downs and an anti-up. The strong nuclear force provides a strong attraction between quarks and antiquarks, so when a proton and antiproton approach to within a distance where this force is operative (less than 1 fm), the quarks tend to pair up with the antiquarks, forming three pions. The energy released in this reaction is substantial, as the rest mass of three pions is much less than the mass of a proton and an antiproton. Energy may also be released by the direct annihilation of a quark with an antiquark. The extra energy can go to the kinetic energy of the released pions, be radiated as gamma rays, or into down or strange quarks. The other flavors of quarks are too massive to be created in this reaction, unless the incident antiproton has kinetic energy far exceeding its rest mass, i.e. is moving close to the speed of light. The newly created quarks and antiquarks pair into mesons, producing additional pions and kaons. Reactions in which proton-antiproton annihilation produces as many as nine mesons have been observed, while production of thirteen mesons is theoretically possible. The generated mesons leave the site of the annihilation at moderate fractions of the speed of light, and decay with whatever lifetime is appropriate for their type of meson.
This blaze of energy also has a very distinctive "spike". Much like nuclear reactions are @ 1000000 times faster than chemical reactions, matter/antimatter reactions happen orders of magnitude faster than nuclear reactions.
Warships, civilian radiation detectors and astronomical instruments tuned to these frequencies will see a very rapid spike of energy, with the distinctive 511KEV signature. If there was a strike on the target, monitoring of the area will reveal the infrared radiation of the spacecraft, asteroid or moon with a hot spot where the warhead struck. (t should be noted that antimatter weapons suffer the same limitation as nuclear weapons in space: there is no atmosphere to convert the radiant energy into shockwaves. The inverse square law suggests that you actually need to detonate the device very close to the target to cause damage.
Nuclear weapons can be used as compact energy sources to drive weapons effects, ranging from "shotgun" charges which can drive pellets at up to 100km/sec, nuclear shaped charges which can drive streams of liquid metal at up to .03 c and even devices which can drive spindles of plasma at .10 c. Using an antimatter reaction to drive these sorts of weapons will provide a much more compact and energetic device compared to a nuclear one.