A discussion of the possible ways in which various surfaces such as the magnetopause, topology interfaces, and source boundaries (plasma sheet or flux transfer events) can be remotely sensed by energetic particles is presented, together with results of computer simulations of particles in a model magnetopause. The conclusion are that these boundaries can indeed be detected from a distance, but that care must be taken to identify the nature of the particle gradient, particularly if more than one gyro-radius is being employed. The distances to the boundary calculated from the simple absorbing wall model will over-estimate the true separation, but the direction determined from the simple model should be correct.