Does the use of watermarked paper mean an etching or lithograph is authentic/original?
Basically … no. Just because an etching, lithograph, or other type of print is on watermarked paper, does not automatically mean a work is authentic/original. Conversely, the lack of a watermark on the paper does not automatically mean a work is not authentic/original.
You should not go by the existence of a watermark on paper alone to determine if a work is authentic/original. More 20th and 21st century artists use watermarked paper than did the 19th century artists. This is partly because there are more choices of watermarked papers for the later artists to use.
Many prints from the 19th century were done on paper that had no watermark. Often later 20th century reprints of these works can appear on watermarked paper. This is done to give the reprints the air of “authenticity/originality”. But in these cases, the existence of the watermark says “this is a later copy … not an original print”. You really have to know what kind of paper was originally used. And it is good to be familiar with whether or not the plates were cancelled or are still in existence for possible reprints/restrikes.
Consulting someone with detailed knowledge of an artist’s work can be crucial in determining authenticity/originality. For prints, a good starting point is www.ifpda.org. You can also consult a catalogue raisonne (an organized catalogue) for an artist’s work to start to build up your own knowledge of what kind of paper an artist used and whether it was watermarked or not. (Serious collectors buy their own copies of these catalogues for their favorite artists.)
So … when it comes to watermarks (or the lack thereof) there is no simple rule to follow. You have to do some research for an artist and a particular work.