Electro-Magnetic Analysis has been recently identified as an efficient technique to retrieve the secret key of cryptographic algorithms. Although similar from a mathematical point of view, Power or Electro-Magnetic Analyses have different advantages in practice. Among the advantages of EM analysis, the feasibility of attacking limited and bounded area of integrated systems is the key one. However, efficient techniques are required to localize hot spots, characterized by partially data-dependent electromagnetic emissions, at which a Differential Electro-Magnetic Analysis (DEMA) may be applied with success to retrieve the secret key. This paper aims at introducing a pragmatic technique to localize quickly and efficiently these points of interest.