You can use the GitHub API.
repo = getJSON(https://api.github.com/repos/jshawl/pocket-oauth-php)
forksURL = repo.forks_url;
forks = getJSON(forksURL);
for ( fork in forks ) {
if ( fork.updated_time > fork.created_time ) {
notify( 'The fork - ' + repo.html_url + 'has been updated since it was forked')
}
}