Ideally, the oil temperature for frying should be between 350-365 degrees. So, without a thermometer, how do you know when your oil is ready to go? There are several different methods I've seen and read. Some suggest dropping a kernal of popcorn into the oil, others suggest tossing a few drops of water into it. The easiest, and safest, method I have found is to stick the end of a wooden spoon into the oil. If you see bubbles form around the wood and they start to float up, your oil is hot enough to cook with.
While a drop or two of water will work, a wooden utensil really is safest. I can never stress enough that water and hot oil do not mix.