Although the correlation between soil tests for sulfur and crop response to sulfur is not very high, soil testing is often used as a starting point.  A consideration of nitrogen/sulfur ratios in soils is another common approach.  Either way, following up with a tissue test allows you to find out how much of the sulfur is actually getting into the crop.  Sampling the exact part of the plant at a precise time and using a standard method of preparation is key, so check with your local lab for guidelines.