Obviously, you can not ask them directly. But sometimes you can tell whether your client got money or not. One of my client keeps sending me email from his iPhone. So I consider that one a rich client. As for me I can't even afford an iPhone (well, maybe I can afford the phone, but definitely not the service plan).
Take the poll to see whether clients are rich or whether they can barely afford to pay the designer.
What's it matter? You set the prices and they'll either pay or not. Doesn't mean there rich or skint.
I've got a iPhone, had one since they first came out and I wouldn't class myself as rich.