If you don’t have a kitchen thermometer, you can absolutely still make Tanghulu.  In order to reach the correct temperature of 300 degrees F, the sugar and water mixture needs to have thickened to a syrup, which takes about 10 minutes at a good boil.  Test for readiness by chilling a metal utensil in ice water, then dip it as you would your fruit.  If the sugar dries immediately then you’ve reached the right temperature and you’re ready to start making Tanghulu.