How can I differentiate between a 'Message' update and a 'Callback Query' update? (Telegram Bot API) – Telegram API

Sorry if my question gets too messy, I’m new here, so, any advice is welcome.

How can I differentiate between a ‘Message’ update and a ‘Callback Query’ update?
I’ve managed to make an inline keyboard, but when I use it, the bot just hangs, he doesn’t reply anything. I did a little bit of research and found this question, which helped me understand the problem, but not much else.

My bot uses something like this right now:

// read incoming info and grab the chatID
$content = file_get_contents("php://input");
$update = json_decode($content, true);
$chatID = $update["message"]["chat"]["id"];

switch($update["message"]["text"]){
    /* insert magic here */
}

So, this code can handle Messages, but not CallbackQueries. If I wantew to handle them, I could use something like this (based on the other question’s answer):

$callback_query = $update["callback_query"]
/* same as above */

But how can I check whether it is a message or a callback query?