
Assuming you are refering to standard POTS or ground start lines:

If you are looking at loop start lines under idle conditions, the RING
conductor is the one with approximately -48 to -52 vDC with respect to
ground while the TIP conductor is at or very near ground potential (be sure
to reference the telco ground when taking your measurements).  

If you are dealing with ground start lines under idle conditions, the RING
conductor will be the one with approximately -48 to -52 vDC while the TIP
conductor would look like it's floating (you may see some potential from
line capacitance it will bleed off over time).  Remember to use the telco
ground as your reference when making measurements. 

