Вы можете сделать что-то вроде этого:
Dim Criteria As String
Dim NotFound As Boolean
Do
RandomString = GetRandomString(StringLength, False, True, False)
Criteria = "[YourField] = '" & RandomString & "'"
NotFound = IsNull(DLookup("[YourField]", "[YourTable]", Criteria))
Loop Until NotFound