I have a shapefile, imported in PostgreSQL using PostGIS when this shapefile is published in GeoServer it didn't show any layer.
I had used EPSG:4326..
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The shapefile should have a .prj file which contains the CRS information. It does not necessarily contain the EPSG code itself, but you can open the file with any text editor, and insert the content in your question.
spatialreference.org is an online database with most known coordinate refrence systems.