Antique copper and brass sometimes can be tricky to buy. Especially when the piece becomes so tarnished or oxidized it's difficult to decipher between the two. Copper was usually used for cooking purposes and the designs were usually simpler than brass pieces. This is a general rule not a sure thing!!
A lot of antique brass has makers marks like silver. These marks help identify the age and the country. This is not always the case, as with silver. Some brass may be signed as I have seen with french brass. Makers marks can usually be found on the bottom or on the handle. Take a picture of the makers mark and than look up on a reputable site such as http://www.oldcopper.org. Quality brass can be determined by the weight of the piece. Some pieces will be light due to the the way they are made. I have a beautiful planter that is lighter than other planters. This is due to the cut outs in the brass. This would be impossible with thick brass. So understanding the piece is also very important to determine the quality.
Copper on the other hand is very different. Weight is very important, but there are many more factors that need to be considered. Seams that are dovetailed are pre 1900. Seams that are harder to find can determine quality. This means more time was put into hammering out the seam. Usually the seams can be found along the side and about one or two inches inside the bottom where the copper was wrapped around the bottom. The process is so labor intensive it would it would not be cost effective to copy. American copper pieces are not identified by hallmarks, but by the makers name and sometimes the place where made. These stamps are usually found around the lip or side.
Many antique dealers suggest that cleaning antique brass can devalue or damage the piece, and removing the patina on investment pieces is guaranteed to decrease their value. Soaking the item in vinegar or lemon juice is a gentle way to clean severely tarnished items. Caustic acids or ammonia must be avoided, as they erode the metal and destroy etching. I suggest using them sparingly on pie that are heavily oxidated. Commercial cleaners can also be used, but be careful of polishing too hard or too often or the copper may leach out of the top layer.
Copper and Brass can add so much to a homes decor. Now with the comeback of brass, knowing a few facts can help the consumer avoid mass produced pieces that hold little value. Antiquing can be lots of fun when a little knowledge is used to buy the perfect piece!!