A camera that's offline will look something like this on the app's Cameras screen:

  

If you try to watch the camera's feed while it's offline, you'll get something like this:

A camera that's offline will look like this on your Cameras page:

If you click the icon in the middle of the blank preview image, you'll get more detail:

If you happen to be watching a camera's feed and it loses connection, you'll get something like this: