VBA To Determine If Workbook Is Open

I cannot cut and paste from my working computer to the internet access computer.

This code is intended to check for a workbook already open and if it contains a particular worksheet. This function is called from a routine that reads a master workbook then opens a PCM workbook. The PCM workbook might already be open.

WB_PCM is declared as a workbook and the name is set. WS_PARAMETERS is declared as a string and its name is set to an existing worksheet. The workbook and worksheet do exist, but are not open.

This is the call to the function:

Code:
WS_exists = Is_Workbook_Open( WB_PCM, WS_PARAMETERS_NAME )

And that code is

Code:
Public Function Is_Workbook_Open( workbook_ref As Workbook, sheet_name As String) as Boolean
Dim check_sheet as Worksheet
On Error Resume Next
If( workbook_ref Is Nothing ) Then
Is_Workbook_Open = False
Else
Set check_sheet = workbook_ref.Worksheets( sheet_name ) ' < error

[code].....

During this time the master workbook is open but either not in focus or is minimized within the Excel windows. If I select that worksheet and select a cell in it, then this code runs. What should I do different to get this code to check for WB_PCM being open and being able to handle it when either open or not open.