How to tell the difference between genuine paintings and reproduction prints:
Both watercolours / watercolors and prints tend to be purchased ready framed, behind glass, which makes direct inspection difficult without removing the image from the frame. Meanwhile, reproduction prints are sometimes given a textured, see-through finish, to make them look like genuine oil paintings. Both these factors make it difficult to tell the real thing from a fake.
However, whether an imitation watercolour or imitation oil painting, you can detect a reproduction with nothing more than a humble magnifying glass, or better still, a camera with a macro setting. If you focus the camera or magnifying glass on a print, the picture will 'break up', revealing that the picture is actually made up of tiny, printed dots - that is to say, not paint. If using a camera set to macro, a resulting photo will reveal this more clearly.
If you are looking at a genuine painting, the image will continue to be viewed as solid paint, even under a high magnification. This technique allows you to inspect a painting, even without removing the covering glass. In the past, I have been sold a print as a painting on eBay and have used close up photographs of the dots as evidence for a refund claim. This evidence is incontestable and should result in a prompt and full refund.
Happy and safe art bidding and buying!