I have had lots of guitars with finish cracks most of them Epiphone or Gibson. You can tell if it is a finish crack by looking at it in a shallow angle with a good light source behind you. A finish crack will only be as deep as the finish is thick and ends where it meets wood and properly lit from a shallow angle that becomes quite obvious. Finish cracks can propagate with no effect on the instrument beyond aesthetic.
It does not matter if the instrument was dropped or not. If you have a wood crack it will propagate over time and if serious will break so play on before it does. One of the problems with Les Paul's is that the thin or low profile neck is not anywhere near as strong as the thicker neck. I think that is called a 'C' neck.
The physics of this is actually quite simple: all materials have a coefficient of expansion and a yield point. Stress applied below the yield point does not affect the material. However past the yield point is the point of plasticity where the material permanently deforms; past the point of plasticity is the fracture point which is a fancy name for a crack and it is permanent.
Some neck cracks are repairable some are not. A someone earlier said composite necks would be preferable. Other than being stiff, straight and holding the frets in place the neck has no effect on tone.
Finally all materials have different rates of expansion and contraction with temperature change. Rapid temperature change is real bad and can cause cracks of many kinds because each material will change dimension (shrink with cold, lengthen with heat) at different rates and this is exacerbated by rapid temperature which will cause problems at the interface between these materials. A Les Paul has three interfaces where the finish meets the binding of the neck. Look there for problems.
You can no more undo a crack than you can return to yesterday (this is called irreversible).
To lower the tension in the neck (the pull force along the strings) play with the lightest gauge string possible ("Boy why you working so hard?" is what BB King said to Billy Gibbons on string gauge, Billy went light and never looked back) and avoid open tunings that raise tension above standard tuning. Open E is particularly bad as is Open A. If you need open E drop down to Open D and put a capo on the second fret. Open A is useless as far as I know.
My $0.02
P.S. I have had a lot of guitars with finish cracks. They appear in both the women and guitars we love.
