Adapted from
Ready For Dessert (Ten Speed)
Although I recommend using a candy thermometer, if you don't have one, simply keep an eye on the pot of ginger while it's cooking in the syrup. When the liquid is the consistency of maple syrup and the bubbles are thick, it’s done.
If you're unsure if your thermometer is telling the truth, boil a pot of water with the thermometer in it. Once the water is boiling, if you live at sea level, the temperature should read 212ºF, or 100ºC.