Caller-ID-Display-Service

If you got a multimeter and a phone you can start testing. Check the voltage across the pair, should be 48VDC in US, 60VDC in Europe.
Place a 600 ohm resistor, 5W across the pair and measure the current should be 20 to 40mA (holding current)

While on-hook the Ring will have a negative voltage and the Tip will be grounded through equipment at the Central Office UNLESS the office is a "Floating Battery" type office. In the latter case the Ring will have a negative voltage and the Tip will have a positive voltage. The sum of the two voltages (which are in series) will be about 48 to 52 Volts.
The usual way of doing this is to use a bicolor LED that glows read with one polarity and green when the polarity is reversed. You just put a resistor in series to limit the current. I think the ones I have usre 3.3k, maybe more if the voltage is high. the max current for a phone line is 60 mA, so it would be wiuse to put 3 or 4 diodes in series in each direction to absorb any excwess current above 20 mA or so. But the dissipation of the resistor may be more than a half watt. So use a couple in series or a 1 watter.
BTW, if you plug one of these into a wall jack, the polarity will be correct, but the modular line cord flips the polarity so that it's
reversed at the phone jack. So you have to know where you're testing.