Get a printers loop or some other fairly strong magnifying glass. Offset prints have a pattern of dots (4) colors that creat a semblance of the original, a giclee often has dots too more visible in lighter areas than dark but not patterned. Also at the edge of a print on canvas the print usually ends in a sharp edge an oil may have paint overlapping the edges or possibly not covering the canvas completely at the edges. After all it''s paint and brushes.
An original Oil painting has no dots probably some texture and possibly cracking for one that age. The canvass is probably linen a darker color, stronger and thought to be higher quality than cotton used in transfers, giclee, etc.