It happens in violin shops all around the world every day. A customer phones to enquire about an instrument that has been in the family for generations. They know it is very old and it has a label that says "Antonio Stradivarius, Cremona, 1723" (or Guarnerius, or Amati, or Maggini, or any other impressive name from the history of violin making). What is it worth and what should they do with it?

The initial response will always be: "I need to see it". It is practically impossible to give any advice, valuations or even repair quotes over the phone. Once they get to see the instrument, most violin makers or dealers will have a standard routine - they may look at the scroll (side view then front), examine the f-holes and purfling, hold the instrument at arm's length to look at the outline, then turn it sideways to check on the arching shape, and so on. The order may vary, but one of the last things they will consider is the label.

This is because the labelling on a large number of violins, violas and cellos is often inaccurate, misleading, doubtful or just completely false.

The history of mislabelling instruments goes back a long way. In 1685 the Duke of Modena was petitioned by a citizen who purchased a Nicolo Amati violin, only to find that the label was false and had been stuck over that of a lesser maker: Francesco Ruggieri. Even Stradivari suffered from label abuse - the dates on some of his labels have been altered to make them appear as if they were produced in his "golden period" in an attempt to increase their worth.

But the practice that has caused the greatest amount of confusion originated in the workshops of Germany and France in the late 1800's, where the mass production approach to violin making began. Thousands of violins, violas and cellos were made in a piecemeal fashion by a series of workers. These are now commonly referred to as "trade instruments" and the quality of the finished product varies greatly. To indicate the model or shape of instrument that was used as a pattern, a label was inserted bearing the name of the original - usually Stradivarius or Guarnerius.

At the time these labels were not meant to deceive, since the instruments were obviously new and were sold as such. In fact no particular effort was made to produce an accurate facsimile of the original label and many also contained words like "Copy of..." or "Made in Germany" (and no, the Cremonese makers did not take their summer holidays on the banks of the Rhine). Imagine buying a new Chinese violin today with a Stradivarius label - would you be fooled? Of course some deception and antiquing was also practiced, and there have always been (and will always be) unscrupulous makers and dealers.

The problem is that these instruments are now over 100 years old and the practice of guessing their age gets harder as they begin to show more evidence of genuine aging. Most untrained eyes could not tell if a violin was 100, 200 or 300 years old, so it is natural to think that an old battered violin dated 1721 is probably genuine. Which is why you need to consult a reputable violin maker, repairer or dealer for a definite answer.

Before you do however, there are a few simple things you can check for yourself:

1. Stradivari only used two types of labels for instruments made after 1700. On both forms the first digit of the year (1) was printed, and he hand wrote the remaining three figures. So if the date is all printed, or if "17" is printed, or if it is all hand written it is not genuine.

2. From 1700 to 1730 his label read:

Note the use of the cursive " u " for " v ", and the old form of " s " as " f ".

After 1730 the label was the same except he used the Roman " v " in his name:

3. Almost every old classic instrument has had a new neck fitted when it was changed from early to modern set-up. The original scroll and pegbox were retained, so you should be able to see a graft line where the neck meets the base of the pegbox.

So when you clean out your great-uncle's shed and discover an old fiddle, by all means take it in to the local violin shop for an appraisal - but don't open the champagne just yet.