SRID is a spatial reference identifier. The SRID of WGS84 is 4326. In Postgis, the spatial query is based on the geometries have the same SRIDs. When using the PostGIS Shapefile Import/Export Manager, the SRID is shown.

To check the SRID of a shapefile:

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SELECT Find_SRID('schema_name', 'table_name', 'geom');

To change the wrong SRID to WGS84:

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-- update SRID in the polygon
UPDATE table_name SET geom=ST_SetSRID(geom,4326);

-- or
ALTER TABLE table_name
 ALTER COLUMN geom TYPE geometry(MultiPolygon,4326) 
  USING ST_SetSRID(geom,4326);

Reference

[1] ST_Transform and ST_SetSRID: To project or not to project?

[2] OmniMarkupPreviewer 404