[action]
One found out on his own just by observing me and Batman back in his world. The others...I think Batman probably told them.
See, there are other Robins here, but none of them are me. Apparently, where they're from, "Robin" is more like a legacy than any one person. At some point I... retired as Robin and took up another identity, so other people took over as "Robin."
There's also Batgirl, who's associated with Batman and Robin. So I'm guessing Batman told her too. Or a "me" did.