Dynamic coupling occurs when multiple groups of strongly connected generators are tied together through weak or strained transmission lines. Establishing which machines in a system are dynamically coupled has shown to be helpful in identifying coherent area for aggregation, providing insights into system stability, as well as indications of potential stability limits. One of the most significant factors influencing dynamic coupling is the value of the transfer impedance between the machines. Therefore, this paper proposes that a strong indicator of which machines are likely to be electrically coupled can be derived from direct evaluation of the off-diagonal entries of the system impedance matrix. Estimation of the dynamic coupling in this manner not only has the benefit of only requiring readily available machine and load flow data but being computationally simple.