It was observed that the optical crosstalk probability (CT) of the silicon photomultipliers (SiPMs) of the electronics in all the AMIGA muon counter modules installed at the Pierre Auger Observatory was reduced by a factor of ∼2.68 after its deployment. This effect motivated the study of the optical crosstalk in SiPMs, in particular, the external interface optical crosstalk (EICT). In this paper, two laboratory measurements were performed to evaluate the CT, and a model to simulate the EICT is proposed. We show that combining CT measurements with simulations, the EICT can be precisely estimated. We also obtained the internal optical crosstalk of the tested SiPM and it was (1.27±0.05)%. This study shows that the thickness of the protective window of the SiPM and the optical system to which the SiPM is coupled affects optical crosstalk. We conclude that the EICT is not negligible and for some cases it is the main contribution to the total optical crosstalk in SiPMs. Taking advantage of the CT variations depending on the optical system to which the SiPM is connected, we propose a method to validate the proper installation of the electronics in the muon counters modules of AMIGA.