The Department of Health and Environmental Control (DHEC) tests the ocean water on the Strand, in knee-deep water, once a week from May to October. See an explanation of the process.
They also maintain a Beach Access Guide. On this map, you can zoom in to Surfside Beach, select the little green and yellow circles on the beach (6 testing sites in Surfside Beach), and see the testing results so far this year.
As far as bacteria, the sources can be terrestrial, i.e. heavy rain pushes out stagnant water from the town's ditch system just prior to testing. This "first flush" effect can have elevated bacteria counts. Also, the source can be something in the water: for instance, if a large flock of birds has occupied the water just prior to DHEC testing.
Either way, one single high reading is not indicative of a water quality problem. If there is a chronic problem or a sewer break, you would have consistently high readings, and steps would have to be taken to repair the issue.