I'm assuming that you don't have the equipment to measure frequency accurately. So without any other information I would pick the value in the middle of the range. Then I would construct an experiment which would determine if the value was fast or slow. An experimet like a one hour timer with an observable output, like an LED, or a buzzer.
Depending on the outcome you would know that the value was above or below the current value. The new range would be half the size of the original, and again pick the middle value and repeat the experiment.
This technique is called a binary search and is quite efficient in establishing an unknown value.