How to detect the time signal convergence automatically

I am working on the FDTD solver coding. You know, when using FDTD to calculate the S parameter, we should stop the simulation when the time signal converges. As a solver, it should have the ability to automatically detect the convergence and stop a simulation at convergence. Thus users do not need to guess how many time-steps they need to run simulations. Many commercial softwares can do that way such as Fidelity. Anybody knows how to automatically detect the time signal convergence in FDTD? Thank you very much in advance.