[Shadowysn]

How do you get the info structure of a user message without trial-and-error?

I want to send a user message that is specific to one or two games.
But with the lack of info and my disliking in having to do extensive trial-and-error, usually I'd try to find info about it online or use a hook.

So far there haven't been any successful findings about it online.
I haven't used a user message hook yet, however since I read that functions that print information to either the chat, console, or hint messages don't work in those hooks, is there any other way?

EDIT:
I meant something like this:

PHP Code:

Handle usermsg = StartMessageAll("UserMessageThatIsNotReal");
if (usermsg != null)
{
    // code to pass on here
    // Let's just assume that UserMessageThatIsNotReal's info goes like this:
    // integer - determine the number of players that get chosen by the game
    // integer - what client index to start from
    // It would probably go like this:
    BfWriteByte(usermsg, 5);
    BfWriteByte(usermsg, -1);
    EndMessage();
    // I want to know the values and the assortment of them for a user message in L4D2 called 'Splatter' but
    // I have no idea what the assortment of that usermsg is, and how many values I have to pass
} 


And how did usermsg all tie to EngineVersion or a specific feature the game contains?