Topological constraint theory has been extensively used to describe how the composition and structure of glasses and glass-forming melts control their properties. This approach relies on an accurate enumeration of the topological constraints acting in the atomic network. Such direct enumeration is challenging since constraints can be active or thermally-broken depending on temperature. Here, based on molecular dynamics simulations, we present a generic method aiming to predict the onset temperature below which constraints become active. We illustrate this method by considering the example of a series of binary calcium silicate glasses. We find that inter-polytope angular bond-bending constraints are associated with a lower onset temperature than intra-polytope angular constraints. Based on this, we show that the differing values of these two onset temperatures largely govern the glasses’ fictive temperature.