Lorentz Reciprocity principle is often used to describe electrical networks and reception/radiation properties of antennas residing in a linear, time-invariant, and symmetric medium. In its reaction integral form, it is usually conceived as a mathematical tool to prove electromagnetic relations. However, reciprocity, more than a mathematical tool, can be used as a powerful alternative to convert a penetration problem into a radiation one for numerical computations and measurements. We review the reciprocity formulation and show simple steps on how to apply reciprocity to penetration problems. Numerical calculations for a wire probe (antenna) inside missile-like structure are carried out for both radiation and its reciprocity formulated penetration problems, and it is shown numerically that results from both methods are identical. One of the advantages of this indirect formulation is that the radiation properties of the structure can be easily measured contrary to the direct measurement of the penetrated signal inside the structure.