I feel that the idea involving transformers, while nice for a one-off job where a quick and not very elegant solution is required, but not at all suited to this job.

This will require some basic knowledge of op-amps, and a bit of testing time, but will be very cheap.

Wind a coil of thin wire around the phase wire of the circuit you want to test. As you increase the number of turns, you should be able to detect an ac voltage on the coil when the phase is active.
(You will need to do a bit of testing to determine a good number of turns)

When you have a decent ac voltage (around 2V or so) coming off the turns of wire, put this through a single diode to make basic DC.

Set the op-amp up as a comparator (method should be in the datasheet for the op-amp) and put the output of your coil into the comparator input. You will need to make some kind of time constant with a capacitor and a resistor to compensate for the DC signal appearing and disappearing with the mains cycle, but this again should be in the datasheet.

The output of the comparator will be very close to its supply rails, so if you supply it with +5V then the output will satisfy most logic inputs.

An LM358 is a cheap and easy to use op-amp that will do the job nicely, and can either be made to do 2 comparators, or can be made to buffer (follow) the input (which is a better idea).

Using the datasheet examples from the LM358 I drew a small circuit diagram which i will post to a website and put the link here soon. It is pretty basic and will need some testing.

In the diagram, the value for the resistors came straight from the datasheet but will probably need to be changed for your application. Also there will be a small delay in the output switching, so I hope that is no problem for your design - this should be around 100ms, or the time for a couple of mains cycles.

Hope this helps,
Craig Bellhouse.