string fieldName(int fieldno);

The function fieldName() returns a name of a field by index.
If the index is out of range then the function returns NULL.

If dbf file contains non-ASCII symbols, you have to use a similar function and provide a code page:
string fieldName(int fieldno, EncodingInfo e);

    // your code
    for (int i=0; i < dbf.fieldCount(); i++)
        Console.WriteLine( dbf.fieldName( i ) );
    // your code