While I highly recommend using a candy or food thermometer (they’re one of my favorite kitchen tools - super cheap and so great for making marshmallows, caramel, and for testing if meat is done!) you can always test to see if it’s at the correct using the cold water test.  To do this, fill a glass or bowl with cold water.  When the syrup starts to turn a slight golden yellow test it by dropping a small spoonful of the sugar syrup into the water. If the sugar syrup is still pliable when you take it out of the water, the sugar syrup needs more time.  Once the tested ball of syrup come out of the water hard enough to crack it’s ready. This is called the hard crack stage of candy making and correspond with about 300-310℉ you’re looking for on the candy thermometer.