At this point in time, pilot reports and the area forecast are your best source of tops.  You can also infer actual tops by going to the Imagery tab in ForeFlight and selecting Infrared.  These static IR satellite images provide you with cloud top temperature.  Basically, the colder the cloud top, the higher the clouds.  While there are important limitations, simply find the height of that temperature shown on the static IR image using our winds and temperatures aloft by using the Winds tab while looking at airport weather.  You can also use the FBWinds that are generated during a file and brief.