The ForeFlight graphical weather briefing includes the Cloud Coverage graphic. This image displays cloud bases and tops along with any icing and mountain obscuration AIRMETs. The Cloud Coverage graphic can be found by retrieving a briefing on the Flights page. Navigate to the Forecasts section > Cloud Coverage.
PIREPs provide another source of Cloud 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 winds that are generated during a briefing.
In flight, if ForeFlight is receiving XM weather through an SXAR1, a Cloud Tops layer will be available on the Maps page.