How to know what shape of a guitar neck should fit your hand?
I've heard that there are four kind of shapes:
It really doesn't matter what it is called. As Tim points out, even if you find a C profile, it may be of a particular radius, or a variable radius.
You may prefer a D in a wide radius, but a C in a smaller radius? Or you may want to look at a V or even one of the stranger profiles like the amazing Strandberg neck which works really well if you use a classical hand position:
I have Ibanez models with various necks, my favourite has a 250mm fretboard radius, but my Warlocks have a thicker D shaped neck, and the Gibsons have a 12" radius. And they all play well - I like how they all feel.
So don't necessarily worry about it. Just practice with them in the shop and buy one that you like. You may like a different type later so buy that as well.
It's not a coincidence that the correct number of instruments a guitarist should have is one more than they currently have :-)
The only way to know is to experience playing them. Whilst there may be names for the neck profiles, hand profiles are more difficult to classify - although medical students may have some ideas...
There's also the depth of the neck to consider, at different points along it, and the width often varies instrument to instrument, maker to maker.
Then, there's the radius of the fingerboard, and the kinds of frets, all of which combine. So it's not a case of 'I prefer a D- shaped neck'. That's maybe a starting point, not a be-all and end-all. And- after hating a U shape for a month or two, your hand may get to prefer it!