
This question comes up more often than you might think, and almost nobody asks it outright — because it sounds like an accusation. It is nothing of the sort. I sell both: original paintings and watercolours, and giclée prints. So I have no reason to praise one at the other's expense.
Most reliably by the surface, seen at an angle with light falling from the side. An oil painting has texture: brush marks, thicker and thinner passages, sometimes distinct ridges of paint. A print is flat — even if the paper has a structure of its own, that structure is uniform across the whole sheet and does not line up with what the image shows.
With watercolour I look at the edge of a colour wash. A real watercolour has uneven pigment settling at the edges and small runs; the paper is slightly buckled where it was wet. A print has a clean edge and flat paper.
The third thing is the signature. On an original it is made with the same material as the work — paint, pencil — and sits ON the surface, so at an angle it catches the light differently from the rest. On a print, a signature copied along with the image is part of the print and looks exactly like it. A print may carry a separate, genuine signature added by hand in the margin — that is an honest arrangement, but it is still a print.
In paper, inks and edition size. A giclée is printed with pigment inks on archival paper in a limited number of copies. A poster is made with dye inks on ordinary paper, in an unlimited run. The difference shows over time: a poster on a wall near a window starts to fade after a few seasons, while a pigment print holds its colour for decades.
When you want a particular image whose original is no longer available or beyond your budget. When the work will hang somewhere exposed to damp or accidents — a kitchen, a small child's room, a rented flat. And when you are buying a gift without knowing the recipient's taste: a print for the price of a dinner is a different decision from an original costing thousands.
Three things, and any honest seller will answer without hesitation. Whether it is an original or a print. If a print — on what paper and in what edition. And whether a certificate comes with the work. An evasive answer to any of these is itself an answer.