The determination of threading dislocation (TD) densities in c-oriented GaN heteroepitaxial films is of drastic importance as they strongly influence the physical properties and device performance. The goal of this study is to assess different imaging techniques for routine determination of TD densities in a materials laboratory. Transmission electron microscopy in plan-view orientation is very accurate to determine TD densities above the mid-108 cm−2 range. But it is a time-consuming and destructive technique. Our results show that cathodoluminescence does not give the true TD number and should therefore not be used. The most adapted technique is atomic force microscopy (AFM) which images the pits associated with TDs at their termination on the surface. Appropriate annealing processes may be required to extend the use of AFM to low TD densities or to reveal all TDs in the case of molecular beam epitaxy grown GaN. With such annealings, AFM imaging may be used for measuring TD densities from 106 to 1011 cm−2. Moreover, AFM (and eventual annealing) is easy to implement and can be considered as a non-destructive technique.