How can I stop ODE with an 'event' function?

I'm using ode15s to solve a system of equations. i apply a sinusoidal load to my system, and i would like to stop my simulation once my system's response is sinusoidal too (when the transitional part of the response is damped)

I was thinking to create a function that compares the last two maximums of my response and stop solving when the difference between these maximums is inferior to a certain value. The thing is, I don't have any idea of how to do this

Any help would be greatly appreciated. Thanks.

Chris