Path Exists will only work if you are looking for a specific file. What you will want to use probably is GetFiles() and that will give you a list of all files in that folder.
Directory.GetFiles(folderpath)
That will give you an array so that you can filter or take its count
In addition to the solution by @ClaytonM which is the way forward, you can check when was the file updated using
Directory.GetLastWriteTime(item.ToString).ToLongDateString
while iterating through the array of file items. This will tell you whether there is a file created on specific month.