How can I determine if a file is empty?

How do I determine if a file is empty? The file is opened by a C program running on the Windows platform. I want to open a file in append mode, and, if empty, first print a header to it.

// Open CSV & write header
report_csv = fopen("SNR.csv", "a+");
if (!report_csv) {
    fprintf(stderr, "Unable to open CSV output file...");
    return -1;
}
if (!ftell(report_csv)) {
    fprintf(report_csv, "Column A;Column B;Column C\n");
}
// ... print data to file
fclose(report_csv);

I was expecting ftell to return the current file size if the file was not empty, which happens because the code above is looped.

However, ftell always returns 0 and the header is printed multiple times.

I know I could fopen it with r and use fseek/ftell/fclose and then fopen it again with a+, but I think it's possible to do this without opening and closing the file multiple times.