As with all candy, fudge requires constant attention to consistency. Not enough mixing or beating and the fudge will be runny; too much mixing and it is likely to freeze in mid-stir while it's still in the pot. Knowing when fudge is ready is an art that candy makers have perfected through years of trial and error.
Bring the fudge mixture to a boil, stirring constantly. As with any candy, stir vigorously while it's on the stove or your mixture will spot-harden, freezing your spoon in mid-stir.
Drizzle a little of the boiling fudge candy mixture into a cup of cold water.
Watch to see if the drizzled mixture collects to form a soft ball. When it does, the fudge is the proper consistency to remove from the heat.
Beat the fudge candy as per your recipe's instructions.
Watch the luster of the fudge candy as you beat it.
Stop beating once the fudge's consistency firms and it loses its gloss, taking on a dull or matte surface appearance.
Pour the fudge candy into a baking pan and refrigerate as per your recipe's instructions.
Lightly press on the fudge periodically with a clean index finger. The candy needs to be firmer each time you test it.
Wait until the fudge has no give at all when testing it with your finger. At this point the candy is properly set and has a consistency ready for cutting.