The pegs are relatively simple to see, assess and understand. Quite simply they are the main tuning part of most violin family instruments.
They are usually made from one of three woods: Ebony, Rose Wood Box Wood, of course, like anything, there are exceptions to the rule.
The peg shaft is tapered, which allows for the peg to go deeper into the reamed holes as they wear out overtime at the same time remaining tight enough not to slip. If the reamed holes and shaft do not match then there will be gaps that make tuning or holding a tune hard to do.
If the hole that the string goes through is too close to the peg box wall then the peg will probably not be able to go in deep enough to remain tightly secured for tuning.
In some cases pegs are cut poorly and end up with an egg shaped shaft. This can work but is, of course, not ideal. That tends to happen in this case is that the tuning becomes irregular and difficult and in some cases, because of this, cases the strings to snap.
When pegs are really tight and do not turn easily this can be remedied by way or two different methods: the one is to use a pencil, ideally 2B, where you colour in the contact areas and holes or there is the age old French method where you first apply good old fashioned soap to the peg and there after chalk to stop it from over slipping.