Think about this hypothetical asteroid made of anti matter. Suppose we plan to land on it. How can we detect whether it is made up of antimatter, or regular matter from a distance?
If we plan to land on it, it must be fairly near Earth. The solar system is made of matter, not anti-matter. In particular, the Sun gives off a stream of particles, the solar wind. This contains largely electrons, protons, and alpha particles.
An anti-matter asteroid near Earth would be bathed in these particles. They would annihilate, giving off gamma rays.
Astronomers pay a lot of attention to gamma rays. For example, the Compton Gamma Ray Observatory was in orbit from 1991 through 2000. If an asteroid near Earth was giving off gamma rays, we would already be aware of it.
It isn't like there might be anti-matter somewhere nearby, and we just haven't seen it. As far as we know, matter and not anti-matter is everywhere in the entire universe. One of the deep unanswered questions in physics is why?
If there was an anti-matter asteroid nearby, it would be the most interesting object in the solar system. We would have already sent missions to study it up as close as we could without being destroyed by radiation.