Internet Service Providers (ISPs) need to balance multiple opposing objectives. On one hand, they strive to offer innovative services to obtain competitive advantages; on the other, they have to interconnect with potentially competing ISPs to achieve reachability, and coordinate with them for certain services. The complexity of balancing these objectives is reflected in the diversity of policies of the Border Gateway Protocol (BGP), the standard inter-domain routing protocol. Unforeseen interactions among the BGP policies of different ISPs can cause routing anomalies. In this work, we propose a methodology to allow ISPs to check their BGP policy configurations for guaranteed convergence to a single stable state. This requires that a set of ISPs share their configurations with each other, or with a trusted third party. Compared to previous approaches to BGP safety, we (1) allow ISPs to use a richer set of policies, (2) do not modify the BGP protocol itself, and (3) detect not only instability, but also multiple stable states. Our methodology is based on the extension of current theoretical frameworks to relax their constraints and use incomplete data. We believe that this provides a rigorous foundation for the design and implementation of safety checking tools.