The easiest way to identify a watercolour is to look for an almost transparent paint layer, which has a perfectly flat surface, with very slight evidence of brush strokes, or pigment on the surface of the picture.
Also consider how the picture is framed, and the material the picture is painted on.
Watercolours are normally framed under glass to protect the delicate paintwork, which can be badly damaged by contact with moisture and dirt.
They are normally painted on a semi-absorbent material like paper or card, and the paint has a transparency not seen in oil paintings.
In fact, you can often see underlying pencil marks made when the artist was composing the picture showing through the paint layer.
Watercolour paint is made from pigments mixed with a water-soluble base, and is applied quickly but carefully, as overpainting of flaws is usually impossible.
The paint layer is normally very thin, although there are some forms of opaque watercolour paint such as gouache. Paintings executed wholly in opaque watercolour paint can look much like an oil painting.
Sometimes an artist adds touches of opaque watercolour to highlight certain elements of a watercolour, and these richer touches of paint should be clearly visible to the naked eye.
Watercolours are relatively fragile, and are vulnerable to sunlight, damp, dust, and contact with glass surfaces.
Note: this information is only intended as basic advice, and not as a definitive guide to identifying different types of pictures.