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Poita is more or less right. On any day the sun rises and sets at the same delta azimuth from north. Only on the solstices will they be truly east and west. (Even those statements are not exactly true. The Earth's position changes during the day but the azimuth change is within any margin of error in your measurements.)
A line between points equidistant along the shadow say 4 or 5 hours before and after solar noon would be very close to east-west and avoids objects blocking your horizon. Good enough for most purposes.
Alternatively, make up a 3-4-5 triangle and use that to give you a line at 90 degrees to the solar noon shadow line.
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