A method for characterization of the isomerization dynamics from classical trajectories is presented. A measure function describing the topological distance between two clusters of atoms is first developed. Next, this measure is used to identify the regions of the potential energy surface visited by the trajectories. Unlike the commonly used techniques such as simulated annealing or quenching, the proposed method does not require repeated treatment of the trajectory and can be safely used to study the isomerization dynamics of large systems, especially those of monatomic clusters.