I stash some metal spoons in the freezer when I turn on the stove. When the caramel is looking about ready, I stick a spoon in the pot and pull it straight back up. This will cool off on the spoon and you can get a feel for how thick it will be once cooled. Other people will smear a bit on a cold plate and see how it tracks. Just remember the caramel is suuuuper hot - don't get burned.