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: 
1. using file extension is not reliable.
2. the best libary you can use is <https://github.com/mono/taglib-sharp> it detects most of the common file types. maybe you just want mp3, so you can extract any mp3 related class.
3. a simpler library you can use is <https://github.com/judwhite/IdSharp>
C# code:

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

```