We often handle paintings that are undated so need to use our judgement in determining how old they are. It's not an easy task but there are several clues that help us to form an educated opinion.
For example, if a painting is on canvas, the appearance of the verso (the back) can provide some telling information.
Here are some examples of the backs of canvases that we've had in the collection over the past few years. Most of these were dated, aside from the early works, so the ages are verifiable.
Most 17th-century paintings are relined. This is a process of attaching a newer canvas on to the back of the older one to preserve it. Both of these examples are relined but still have the original stretcher bars. Note how the bars are very roughly hand-cut and often contain the original hand-wrought nails.
Generally, the back of an 18th-century canvas (if unlined) will have darkened due to the effects of time. The stretcher bars tend to be quite rudimentary too.
Many canvases from the 19th century have been relined, such as this one from 1838. In this example, the stretcher bars are original and quite basic in their construction.
Here are some examples of unlined 19th-century canvases.
Note how this is lighter due to its preservation under backing paper.
The following two canvases are both from Germany during the war. Most professional canvases from the 1940s are a little darker but during this period artists were painting on anything they could reasonably use due to a shortage of materials.
Some canvases have a stamp to identify the supplier, which is useful when trying to ascertain its age. Over the years, suppliers updated their stamps regularly so each one has a particular time frame. The National Portrait Gallery keeps a very useful record of how the stamps have changed over time and its database can be found here:
Here are the references for the main British suppliers.
Although always bear in mind that an artist could theoretically use an old canvas for a modern painting, so do examine all of the clues before forming an opinion.