Check if file is open

So I have five workbooks. Four are for Users, one is a Master. The Master opens the other four, one at a time, and copies a portion of the data from them, and pastes it into the Master. However, utilizing an application called Dropbox, if data is imported from a workbook a User has open at the time, it creates a "conflicted copy". What I need is to have my current code verify the file it is attempting to open is not currently being used. If it is being used, I need the macro to stop in its tracks and provide a message that reads, "Workbook (Filename that is currently open) is currently open".

Here is my code currently:

Sub ImportWorkbooks()
Dim wbData As Workbook, wsMaster As Worksheet
Dim NR As Long, fPATH As String, fNAME As String

fPATH = "C:\Users\Officer\Dropbox\Leads\"
Set wsMaster = ThisWorkbook.Sheets("Sheet1")

With wsMaster
NR = .Range("B" & .Rows.Count).End(xlUp).Row + 1

fNAME = Dir(fPATH & "Leads*.xls")

Do While Len(fNAME) > 0
Set wbData = Workbooks.Open(fPATH & fNAME)
Range("B12:S1000").Copy .Range("B" & NR)
wbData.Close False
NR = .Range("B" & .Rows.Count).End(xlUp).Row + 1
fNAME = Dir
Loop
Range("B12:S1000").Select
Selection.NumberFormat = "@"
Selection.HorizontalAlignment = xlLeft

End With

Any help would be appreciated!

That works if the user wb is open on the pc I'm working on, but not if they have it opened at another station. So halfway there i think. Is there even a way to test if the file is open on a pc other than your own (since we are all accessing these files from a shared folder?

I'm using software called Dropbox, which allows for a sort of shared folder without being on a local network (or using a common server). But its not a shared folder in the traditional sense.