First off, even if it has a tag inside that says "Stradivarius," don't quit your day job. Most of Stradivarius's violins have been accounted for, so chances are that you don't have one. Particularly if the tag also says "Made in Czechoslovakia"! The tags inside of violins are notoriously inaccurate because they are so easily counterfeited.
Before you even take the violin to the music shop for an appraisal, look at its condition. As you are facing the violin, with the strings up, check the lower right-hand "bout" or quadrant of the instrument's front. If there is a crack there or on the left lower bout of the back, then you have a nice wall ornament, not a playable instrument. The reason is that there is a little post of wood that is wedged between the front and the back of the instrument in that region. The purpose of the sound post is to help transfer sound to all parts of the instrument as it is being played. But if the violin is cracked in the region of the sound post, it can't be fixed.
Suppose the violin is sound--no cracks--but you notice that the top is separated from the sides. PUT AWAY THAT TITE-BOND! Violins are put together with hide glue, which separates when heated. This is the way that violins get repaired--the luthier heats the seams, takes apart the instrument, does the repairs, and puts it back together with hide glue. If you put a modern wood glue in there, the violin will become impossible to repair and basically worthless.
And while we are at it, even if the varnish is cracked or in some other way messed up, don't touch it. A new varnish job will make a nice old violin worthless. The art of creating a varnish recipe, which is often a deep dark secret in violin-making families, has a lot to do with how the instrument sounds. A nice new finish might make the instrument look better--but again, you'll have a wall ornament, not a playable (or sell-able) musical instrument.
There are some clues about quality that might be important. On the body of the violin are two f-shaped holes. On good violins, these are perfectly symmetrical and beautifully carved. On cheaper ones, they might be a little crude looking.
The fingerboard on a good violin should be dark black ebony. On cheaper instruments, they use other types of wood and stain it black--you can see the stain wearing off.
The neck and the scroll may or may not be original to the violin. But on a good instrument, the scroll is beautifully carved out with smooth, clean lines.
To get an appraisal, go to a reputable violin shop. In fact, if you think there might be some real value to the instruments, get a couple of opinions.
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