Not clearer but probably the fastest solution:

if ((isset($array["message"]["action"]) ||
        (array_key_exists('message', $array) && array_key_exists('action', $array['message']))) {
    /* ... */
}

The second check is needed because some values evaluated with isset() will result false despite the index does exist.

Not clearer but probably the fastest solution:

if ((isset($array["message"]["action"]) ||
        (array_key_exists('message', $array) && array_key_exists('action', $array['message']))) {
    /* ... */
}

The second check is needed because some values evaluated with isset() will result false despite the index does exist.