Currently broadcast message is only separate by "info" icon instead of the presence icon in the chat tab. But if you then start talking to a person, who has send this broadcast, then this icon changes to presence bubble and you can't say which message was a broadcast. There should be some additional indication in the chat window (and also in the history) for the broadcast messages. It could be the same "info" icon showing prior broadcast messages, or simply "(Broadcast)" string prior the message (or a color background, say light red, light green, etc.). Or this message can be bolded. I prefer "(Broadcast)" (should be translatable in i18n files) and bold.
This is also should be done on the sender part, when another ticket will be resolved, about adding broadcast messages to the sender's history.