Yeah, that's one nice thing about DVD-A: you're virtually guaranteed that it'll be in 5.1. With SACDs, I've found that it doesn't always have that little black box on the back with the white print that says "Multi-Ch", but there's usually some indication. Now that I know, I always check it thoroughly before considering a purchase. And if it doesn't say anything, I'd recommend asking the clerk/manager about refunds if it isn't multi-channel, unless of course, it doesn't matter to you.
cheers,
supervij