How do you know when your soap gells?

I was completing a custom order I've been working on (personalized soaps for Christmas presents; what a fun idea!) and I took this picture to show what it means to 'gel' your soap.

While the saponifaction process is underway, the raw soap batter heats up quite a bit. The heated soap 'gells', starting at the center and spreading to the edges. This causes the soap to darken slightly from the origional pour color, but in no other way changes the composition of the soap.

Some soapmakers prefer not to gel their soaps, and attempt to prevent gelling by putting the raw soap into the fridge or freezer to cool it off. Since soap naturally wants to gel, I go ahead and let it, encouraging it if it doesn't complete itself (as it sometimes will start to cool back down before reaching the edges) by covering it with blankets or placing it in a barely-warm oven.

So, for your reference purposes, the soap on the left is almost completely gelled (with the edges still un-gelled) and the soap on the right has not yet started to heat up.