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`
C# code:

```
bool isMP3(byte[] buf)
{
    if (buf[0] == 0xFF && (buf[1] & 0xF6) > 0xF0 && (buf[2] & 0xF0) != 0xF0) 
    {
         return true;
    }
    return false;
}

```