Personally, I just pop my disc into my DVD player, switch my TV mode to 16x9 (I don't own a widescreen TV), and if the image looks stretched out, it's anamorphic. If it plays as it normally does, it's not.

I don't understand how PowerDVD would be able to check if it's not playing the DVD. Where would it get its information otherwise? I know that if you're playing a DVD, you can go to "Configuration" and go to the "Information" tab and it'll state whether the aspect ratio is 16x9 (anamorphic) or 4:3 (nonanamorphic).