Hi,
All Asynchrounous callback from Unity are using a special Class, which you simply store as a private variable inside your action and check on Update it's status.
in your case, I assume you are using the WWW class right? check out the wwwObject action and how it handles knowing when the www content has been received back from the server.
you need to do that same.
Now, you are mentioning userdata and not directly a www class, so are you using a framework from the asset store or something you wrote yourself?
Bye,
Jean