Actually, radio triggers do a lot of the work for you. First off, the "text" field of the trigger will normally be displayed, unless you overwrite it with the setradiomsg command, so in most cases there is no need to use that command at all.
Even better, when a radio trigger is unable to be activated, it will AUTOMATICALLY hide that radio channel from the player. For example, if the condition in your radio trigger was:
this AND !(alive Enemy1)
Then the radio message would only be shown when Enemy1 was dead.