If you do have a thermometer you need to test its accuracy.
First bring a pot of water to a boil and add your thermometer.
Make sure the bottom of the thermometer is not resting on the bottom of the pot or your results not be as accurate.
Boil the water for at least 10 minutes and note the temperature.
Water, at sea level, will boil at 212° F (100° C).
For every 500 feet you are above sea level the boiling point will approximately be decreased by 1 F.
If after 10 minutes your thermometer does not read 212 F (100 C) note the difference.
When following a candy recipe add the amount of the difference to the temperature the recipe calls for.
For eaxmple my water boiled at 211 F so when I make candy I make sure my candy reaches 1 degree higher that the temperature called for in the recipe. If a recipe reads to bring the syrup to 235 F I will wait until my thermometer reads at least 236 F.