The easiest way to identify an oil painting is to look for raised brush strokes on the surface of the picture. The surface texture should be clearly visible to the naked eye.
The picture may also have been varnished - a shiny layer applied to the surface of the picture to protect the underlying paint, and to bring out the colour of the paint.
Oil paintings are rarely framed behind glass, and the frames used are often highly decorative, and feature various embellishments, including gilding.
They are normally painted on canvas stretched over a wooden frame, on board, or on a wooden panel.
Oil paint is made by grinding pigments in oil to create a flexible and durable medium.
Modern acrylic paint uses the same principle, but is made of synthetic materials that are water soluble.
Oil paint enables both subtle graduation of tone and sharpness of line, and is unsurpassed for textural variation. It is usually applied with brushes or a thin palette knife.
Note: this information is only intended as basic advice, and not as a definitive guide to identifying different types of pictures.