To me, that's like asking how you meet a celebrity. They could just be doing their groceries at the same store as you, because they're people and need to do that, and maybe they're vacationing in your town while their TV series is on hiatus or something. It could also help if you go to convention panels or autograph signings, but you'd need to know when and where that event would be, how to travel there, if there's an entrance fee, who is really going to be there, and so on. Or you could just run into people who look a lot like a celebrity or, maybe, being influenced by someone influenced by a lesser-known celebrity...have a lot of the same attitude as a result, and then keep you wondering, "Wait, was that just ___ I saw??" But you've got obligations and aren't sure or you're shy and you've already lost sight of them so you don't follow.
This metaphor kind of falls apart if I bring up like a friend of a friend puts up a YouTube video that they didn't expect to go viral, but a parallel question would remain: what is celebrity?
Likewise, what do you mean by "dragon"?
You can do particular meditations to get you astral, and you'll find some very strange things over in the otherworld. A dragon might be one of them, but it might not be exactly as you think; but once you've caught the "style" or the Name of it, then you can call it, and if it doesn't mind you thinking it's a dragon, and you can't think of it as anything but a dragon, then it's probably a dragon. Or you can just go around exploring and ask random people you meet where you can see one.
Like with the grocery store celebrity sighting example, the dragon might already be there. Or like the convention example, you might have to go looking around for the astral garden of Hesperides where Ladon guards the golden apple tree, or find a torii dragon gate, or something.