How can I tell if a given path is a directory or a file? (C/C++)

stat() will tell you this.

struct stat s;
if( stat(path,&s) == 0 )
{
    if( s.st_mode & S_IFDIR )
    {
        //it's a directory
    }
    else if( s.st_mode & S_IFREG )
    {
        //it's a file
    }
    else
    {
        //something else
    }
}
else
{
    //error
}