When simulating a tapered waveguide or fiber using the EME solver, the taper will be discretized along the propagation direction using a specified number of cells in each cell group.
One way to make sure that you are using enough cells is to perform convergence testing by increasing the number of cells and re-running the simulation until the results converge, as demonstrated in this example:
Another tool that can be used is the error diagnostics which are returns results in the EME solver region object after running the simulation and performing the EME propagate step. The error diagnostics can return information such as the amount of field discontinuity, and power gain and loss between neighboring cells. More information about the EME error diagnostic results as well as an example showing how to use the results can be found on this page: