A ``rubble pile'' asteroid is defined as a weak aggregate of large and small components held together by gravity rather than material strength. Thus, calling something a rubble pile could mean it is composed of large boulders and/or sand-sized particles.
There is no easy way to determine the internal makeup of an asteroid. Remote sensing techniques can, at best, only probe the top layers of an asteroid's surface. More elaborate methods, such as deep core drilling and sample return, staging and observing a moderate-sized impact event, or setting up an in situ network of seismometers, could provide more diagnostic information, but they would require spacecraft funded at a level comparable to or higher than NASA's current generation of Discovery-class missions. Still, the information we possess today on asteroids, taken from spacecraft flybys, ground-based observations, numerical models, and theoretical advances, is strong enough to make inferences on what these bodies are like. In the following sections, I present a distillation of this data, which I believe supports the view that asteroids are rubble piles.