This may seem a bit simplistic but the first thing to do is ask. No artist should be offended or put out by the question, "Is this an original piece?" Most artists will welcome the question because it now provides a platform for engaging you in talking about their work and the artistic process s/he uses to create the work. Being a trusting soul, I've always started here. However, the part of me that is firmly rooted to the earth, now says you must go deeper.
It's time to put on your best Sherlock Holmes impression and give the piece a closer examination while asking more questions. Don't be shy about standing face to face with the artwork, get close, and then get closer. The thing I love most about visiting museums to see my favorite artists' work is that I can see the brushstrokes that went into creating the masterpieces. I hate it when the museum puts up those ropes. I can't see the total genius behind the work if I can't get close.
The brushstrokes you see in oil, acrylic, and egg tempera paintings should be strokes of color not clear coatings. Unscrupulous artists may touch up reproductions and present them as originals using a gel medium to simulate brushstrokes. Look at the artwork on several angles and in sunlight. There should also be texture throughout that is not uniform as you may see with textured papers.
If you are commissioning a work, the best possible way to ensure you are getting an original, authentic piece of art is to visit the artist in his/her studio throughout the production. This can be unsettling for some artists, but it's your money and your artwork. If the artist is unwilling to permit this, I'm sure there's another talented artist longing for the commission who is more than willing to allow you access!
Bon chance y buena suerte!
vivian leflore mora