Something like this will get you started:
On Error Resume Next
Dim wSheet as Worksheet
Set wSheet = Sheets(1) ' can also be a string, such as Sheets("Sheet1")
If wSheet Is Nothing Then
MsgBox "Worksheet not found!"
Set wSheet = Nothing ' make the worksheet point to nothing.
On Error GoTo 0
Else
MsgBox "Worksheet found!"
Set wSheet = Nothing ' set the found Worksheet object to nothing. You can use the found wSheet for your purposes, though.
End If
This code was based on http://www.ozgrid.com/VBA/IsWorkbookOpen.htm. Look for the DoesSheetExist() sub.
Hope this helps!