//It depends if you want to work with the user afterwards or only check
//if one exists.
//If you want to use the user object if it exists:
$user = User::where('email', '=', Input::get('email'))->first();
if ($user === null) {
// user doesn't exist
}
//And if you only want to check
if (User::where('email', '=', Input::get('email'))->count() > 0) {
// user found
}
//Or even nicer
if (User::where('email', '=', Input::get('email'))->exists()) {
// user found
}