Instructions
Examine the color. Antique brass is a golden-yellow color versus a more reddish hue found in faux antiques merely made to look old. You may need to polish (or ask the owner to polish) a section of the mortar and pestle to see the color underneath.
Study the patina. Antique brass will develop a soft texture and a yellow-brown color. Artificial patina has a rougher texture and a reddish-brown color.
Search for seams. Mass-produced brass is cast in one piece, but the old, hand-crafted antique brass mortar will have a seam, though it will be only faintly discernible since the craftsman will have ground and polished it away. Look for a thin line indicating the seam of the two pieces joined together.
Check for the stamp of a brazier (a brass maker), though these are rare, even in antique brass. They can provide some valuable clues to the age and origin of a piece. If you find a mark, you may want to show it to a knowledgeable antique dealer for further information. If, however, the mark indicates something like "Made in China," it is definitely not an antique.