Question: How can I determine that a particular file (which may or may not have an ".mp3" file extension) is in fact an MP3 file? I wish to do this in C#.

Answer: 
Files often start with a "magic number" to identify the data format. Depending on the format, a file starts with a certain sequence of bytes which is unique to that format. There's no standard to follow so it's not 100% reliable. 

As fvu says, the mp3 magic number is `0x49 0x44 0x33`
```
string[] filePath = Directory.GetFiles(fbdialog.SelectedPath.ToString(),".mp3", 
                                       SearchOption.AllDirectories);

foreach (string str in filePath)
{
    MessageBox.Show("It's mp3 file");
}

```