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if the message is a standard sensor_msgs/LaserScan, there are fields describing the angular coverage of the scan. The ray lengths in the ranges[] array should be ordered sequentially from angle_min up to almost angle_max, sweeping counter-clockwise if you look down from above.

The answer to your question is that, for the standard LaserScan message, angle 0.0 is "forward" in the sensor's own transform frame.