var page = pdfViewer1.CurrentPage;
var pageObject = page.PageObjects[0];

var markedContent = pageObject.MarkedContent[0]; //The MarkedContent collection can contain multiple types of objects, so check them all, not just the first element as in this line.
var optionalcontent = markedContent as PdfOptionalContent; //You need an optional content only.
if(optionalcontent!= null)
{
    var group = optionalcontent.Group;
    if(group == null)
    {
        // optional content may be part of the membershop. In this case, the Group property is null
        group = optionalcontent.Membership.OCGs[0]; //Collection OCGs may contain several groups. I think you need to check all of them, not just the first item as in the example.
    }
    bool isStateOn = pdfViewer1.Document.OCProperties.CheckVisibility(group);
}