You can use a new HTML5 API to check if they give you permission to use the webcam. After all, if they deny permission, they might as well not have a webcam, from the code's perspective.

See navigator.getUserMedia().

EDIT:

navigator.getMedia = ( navigator.getUserMedia || // use the proper vendor prefix

                       navigator.webkitGetUserMedia ||

                       navigator.mozGetUserMedia ||

                       navigator.msGetUserMedia);



navigator.getMedia({video: true}, function() {

  // webcam is available

}, function() {

  // webcam is not available

});