超简单， 通过Range的MergeCells 即可判断。
示例代码如下：

Excel.Application oXL;
            Excel._Workbook oWB;
            Excel._Worksheet oSheet;
            Excel.Range oRng;
            object missing = System.Type.Missing;

            //创建Excel实例
            oXL = new Excel.Application();

            //打开已有的工作簿
            oWB = oXL.Workbooks.Open(@"E:\BookTest.xls",missing,missing,missing,missing,missing,missing,
                    missing,missing,missing,missing,missing,missing,missing,missing);
                oSheet = (Excel.Worksheet)oWB.Sheets.get_Item(1);
            
            // 获取 A1 单元格
            oRng = (Excel.Range)oSheet.Cells[1,"A"];
            bool isMerge = (bool)oRng.MergeCells;
            MessageBox.Show(isMerge.ToString());