How do I check sandbox fields on an entity

You can access Sandbox (or ArchiveStatus) fields by casting the entity. For Sandbox fields:

if (product instanceof SandBoxDiscriminator) {    // Cast the entity 
    ((SandBoxDiscriminator) product).getSandBoxArchivedFlag() 
}

The same can be done for ArchiveStatus fields. Check if it is an instance, then cast it.