you need a volt meter
usually, the dashed line indicates the positive wire-- this should go to the center terminal on the barrel connector. the solid black wire is typically negative and goes to the outside of the barrel connector.
but on expensive parts like the monitor, i always double check the correct polarity before connecting the screen.
when i did my screen, i cut the power cable, then plugged the cig lighter adapter in, so i could measure the wire with a volt meter, and verify which wire the dashed one was..