try {
    //Try to connect using FDO. If you can open/close without exceptions, it's a valid SDF3 file
} catch (OSGeo.FDO.Common.Exception ex) {
    if (ex.Message == "SDF file appears to be version 2.1 or older. Please convert to SDF version 3") {
        //Try using SDF loader. If you can open/close without errors, it's a valid SDFv2 file
    }
}

//while we have the file open, check if it is
//an older SDF (2.1 or older) by reading the first few bytes.
//all this is done in little-endian.
unsigned long value;
fread(&value, sizeof(unsigned long), 1, f);
fclose(f);

if((value & 0x0000ffff) == 0x00002c00)
{
    // 2.x versions
    // Might also include 1.0, but it is not
    //guaranteed to be old SDF since other binary files
    //can have the same initial bytes

    //Note that for SDF3 (SQLite) files, the first 4 DWORDS are:
    //value[0] == 0x00000000
    //value[1] == 0x00000001
    //value[2] == 0x00000000
    //value[3] == 0x00053162
    //But the correct way to check version from now on is to
    //open the Schema database and read the version number, which
    //we do later in this function.

    throw ...
}