Insert doesn't return anything but a success boolean, so I think your only option is to query your table before the insert to find the max id, then count your $data array with count(), and determine the to-be ids with that.

There might be a more semantic way to do this...but I think this will work (untested).

$data = array(
    array('name'=>'Bar', 'lastname'=>'Baz'),
    array('name'=>'Foo', 'lastname'=>'Baz'),
);

$max_id = DB::table('users')->max('id');
$ids = array();
foreach ($data as $i => $row) /* assuming non-associative keys, and that at least one record already exists */
{
    $ids[] = $max_id + $i + 1;
}

YourModel::insert($data);