How to tell one Exception from another? Is there an error code?

Say, I want to handle exceptions differently. For instance, if it's a certain exception I want to show a specific message for a user, but for some exceptions I want to display a generic, "Error has occurred". In the code below, how can I tell one exception from another? Is there an error code somewhere?
try
{
}
catch (Exception ex)
{
 //How to tell if this is "Maximum request length exceeded." exception?
}
...