What you want to do is possible, but the answer will be much better targetted to your application if you provide the following information:

1) Do you mean the signal 0-5V?
2) What shape is the waveform? Sinusoidal, square, etc.
3) Does the signal include any noise, i.e. does it come from an external device, or from a nearby logic chip?
4) What is the approximate frequency of the activated state and deactivated state?
5) What language are you coding in?

Without knowing the answer to any of these, a generic approach might be:

Connect the signal to an interrupt line. Set a timer to wait slightly longer than the period of the activated signal. Start the timer at a low-to-high transition. If the timer overflow interrupt occurs first, the device is not activated. If the external interrupt occurs first, the device is activated.

That is just a starting point for one idea.

- Lee