How can you tell before buying if a widescreen monitor supports vertical orientation?

I should have clarified that when I rotate my monitor, the image rotates by way of the monitor's own program.
That is to say that I do not rotate the image from the video card.
I just turn the monitor, and at a certain point of rotation, much like a tablet, the image rotates by itself.
The necessary software came with the monitor, naturally.

PS: Whatever it costs you to buy something similar now, no matter how expensive it is, at least you'll know that you'll be paying less than I did.
When they first came out, they were a shade under $1,000 and I waited and waited for a special and got one of the last new ones around out of some store near Annapolis for $550, lol... for a 24 incher, and I felt like I'd just gotten the deal of the century.
Dang things were expensive then. I can't remember exactly how long I've had it, but my best guess is 5 years.

Are you even going to use a base or just mount them on a stand?
Bear in mind, that if it's a fairly big screen, working on a monitor in portrait mode, will give you a crick in the neck after awhile.