Chappy

Actually just opening the computer case will not void the warranty from any OEM machine that I know of, but opening the PSU case itself certainly will.

The only reliable way to do it without opening the case is to look at the back of the box and see if it has a sticker on the PSU that states it's ratings. Or as mentioned above, if you have an OEM machine and haven't modified it, you can go their website and find your model and it will show the PSU ratings.

Other than that, it's quick and easy to remove the side of the case and look at the PSU sticker. You don't have to touch anything (usually) as most PSU's have the sticker oriented so that it's easily visible when the case is open.