How to tell roughly when a piece was made.
At the moment there is an a huge rotation in the antiques market. In case you have not heard some of the best buyers are about 30 or younger. They sense an opportunity and are most likely bored with slick catalog furniture. Buyers include designers that like the flexability of placing antiques and vintage pieces. If a client does not like the piece usually it can be returned and replaced. There is no long wait for the piece to be made and shipped. I believe usually these pieces are not so easily exchanged. No matter how it looks it is yours. Another group of buyers is generational. The families have been collecting for a while and they prefer the timeless look of well selected antiques / vintage pieces. It seems a lifetime ago but for many decades these pieces where considered assets. Now it is hard to make this argument prices are roughly about where they where in the 1970’s. Of course there are brilliant exceptions but generally prices are attractive at the moment. A fair number of people are down sizing or moving to the countryside. They no longer need an enormous formal dining table or the pieces that go with it. Generally they seem to be choosing less formal pieces. It is worth mentioning that at least one of the catalog companies is buying more formal decoration. Whether it turns into a trend that includes furniture I can only guess.
So now for the interesting part. There are no hard and fast rules. Always at least a few exceptions. We think about pieces as originals made when the design first became fashionable and there are copies. Some of the copies are very well done and their are rare instances where the copies can be more valuable than the originals as the workmanship and quality is so high.
As a general rule plywood indicates the piece is later. Occasionally it might be used in a repair for an original. Plywood often replaces mirror backs. Sometimes the mirrors where built into the wall without a back or the original back has fallen off. I am not certain when plywood was commonly used but I have seen it on a fair number of late 19th century pieces.
Key holes will tell you something about the dating of a piece. The originals have key holes that are larger and shaped a little like the number 8. They are usually handmade so they vary slightly from one another. One they where machine made the key holes are smaller and have that classic keyhole shape with the horizontal bottom edge being flat. Later in the 20th century keyholes begin to vanish. Clothing is not as valuable and the need to lock things up fades away. The exception here is provincial furniture. Sometimes no locks present or only one. The need to lock things up was not as important and they were not wealthy enough to have many things to secure.
Tool marks are a good way to help you decide a pieces age. Hand cut veneers are thick about 1/8th of an inch. As soon as possible the machine cut veneers where adopted after 1840. They are very thin sometimes approaching the thickness of paper. Saw marks are great. In an older piece you can see that they are irregular. In a machine age piece they are matching / uniform. The surfaces are worked enough to make them look smooth but in reflected light you can see the irregularities. This looks a little like ripples. You can also feel this variation if you run your hand over the surface with your eyes closed. Machine age pieces are perfectly flat and smooth. The edges on more recent pieces are fairly sharp. On an old piece the edges are smooth, slightly irregular with wear and feel good when you run your hand over them. An old edge is very hard to fake or reproduce.
I am certain there are dozens of other pointers but the last one I will list is the feet. On English pieces they are often replaced and will seem fresh. This kind of repair is not unusual and generally most dealers don’t worry about it too much. Repairs to feet are not unusual on French pieces because they are delicate and the climate allows wood worm to very slowly munch on a piece. The worms are fairly common and easily treated. They are quite small and it takes a long while for damage to show up. Because the legs will remain fragile even when repaired and it is difficult to tell how much wood loss has happened you might want to be more cautious. An original old foot will be patinated more heavily as that is where most of the ware shows up. Overtime wood changes shape. It does not get shorter but it does get slightly thinner. The feet and joints are where you will find this.