It's basically impossible to rule out that an image (or anything rasterized) is watermarked.
The best way of ensuring this isn't to test it, but to simply ask the producer whether they would do this. If they say "no" or refuse to answer then don't buy that product. Not even if you had a printer from a manufacturer that had public firmware and driver code could you be sure by just inspecting the code and the printed output.
If they clearly say they don't watermark output then you probably have to trust them or simply not use printers.