マウスドラッグでセル範囲を選択したときに、SelChangeイベントが発生するので、ここでSelectionプロパティを使用して選択範囲を取得した後、GetMergedRangeメソッドを用いて選択範囲に含まれるマージ範囲を参照できます。

◎サンプルコード（VB）

    Private Sub C1FlexGrid1_SelChange(sender As Object, e As EventArgs) Handles C1FlexGrid1.SelChange
        ' 選択範囲の取得
        Dim range = C1FlexGrid1.Selection
        
        ' 選択範囲内のMergedRangeの取得
        Dim mergedRanges As CellRange() = New CellRange() {}
        For i = range.r1 To range.r2
            For j = range.c1 To range.c2
                Dim temp = C1FlexGrid1.GetMergedRange(i, j)
                If Not mergedRanges.Contains(temp) Then
                    Array.Resize(mergedRanges, mergedRanges.Length + 1)
                    mergedRanges(mergedRanges.Length - 1) = temp
                End If
            Next
        Next
        
        ' MergedRangeの調査
        Dim rows As Integer() = New Integer() {}
        For i = 0 To mergedRanges.Length - 1
            Console.WriteLine($"{mergedRanges(i)}")
            If Not rows.Contains(mergedRanges(i).r1) Then
                Array.Resize(rows, rows.Length + 1)
                rows(rows.Length - 1) = mergedRanges(i).r1
            End If
            If Not rows.Contains(mergedRanges(i).r2) Then
                Array.Resize(rows, rows.Length + 1)
                rows(rows.Length - 1) = mergedRanges(i).r2
            End If
        Next
    End Sub

◎サンプルコード（C#）

        private void c1FlexGrid1_SelChange(object sender, EventArgs e)
        {
            // 選択範囲の取得
            CellRange range = c1FlexGrid1.Selection;

            // 選択範囲内のMergedRangeの取得
            CellRange[] mergedRanges = new CellRange[0];
            for (int i = range.r1; i <= range.r2; i++)
            {
                for (int j = range.c1; j <= range.c2; j++)
                {
                    CellRange temp = c1FlexGrid1.GetMergedRange(i, j);
                    if (!mergedRanges.Contains(temp))
                    {
                        Array.Resize(ref mergedRanges, mergedRanges.Length + 1);
                        mergedRanges[mergedRanges.Length - 1] = temp;
                    }
                }
            }

            // MergedRangeの調査
            int[] rows = new int[0];
            for (int i  = 0; i <= mergedRanges.Length - 1; i++)
            {
                Console.WriteLine($"{mergedRanges[i]}");
                if (!rows.Contains(mergedRanges[i].r1))
                {
                    Array.Resize(ref rows, rows.Length + 1);
                    rows[rows.Length - 1] = mergedRanges[i].r1;
                }

                if (!rows.Contains(mergedRanges[i].r2))
                {
                    Array.Resize(ref rows, rows.Length + 1);
                    rows[rows.Length - 1] = mergedRanges[i].r2;
                }
            }
        }