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The easiest way is to invest in a tone generator and probe. http://www.monoprice.com/product?p_id=8134
You could also disconnect one pair at a time and check everything and see what no longer works. Generally the main phone lines are the blue pair, and the orange pair is generally used as line 2.
BTW, it is possible to have both phone and data in the same CAT5 as long as you don't need gigabit.
Non-gigabit (10/100) cat5 eithernet connections only use two of the 4 pairs. So it's possible to use the blue pair for phone and the orange and green pair for data. You'd need dual jack outlets and seperate the pairs at the outlets. Blue pair to center two pins of a RJ11 or RJ12 phone jack on top and the green pair to pins 1 and 2 and the orange pair to pins 3 and 6 of a RJ45 jack on the bottom.
With them all tied together there is no way to determine which goes where. You would not need to disconnect all 8 wires for each, one pair would be fine. Disconnect 1 blue pair at a time and put a toner on it.