Fiddling is something I do when I am not turning wood. Here I am holding a new fiddle and checking it to see if it is any good.
It turns out that is a good one and I get it for one of my newest students. It is better than one that was offered to me for a higher price.
One of the first tings to look for is the use of nice wood. That is easy. Just look. This has lovely "Flamed Maple".
The second thing I look at is to see if everything is made correctly. That takes experience.
The third thing I look for is an even tone from top to bottom. For this you need to play the instrument in several different styles right across the range from top to bottom. Listen very carefully.
The fourth thing I look for is the quality of sound. That takes lots of comparing instruments. You always try as many instruments as people will let you play.