UITableView, how do I know what Section during cellForRowAtIndexPath?

you can use indexPath.section and indexPath.row

The method is called

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath

The indexpath parameter contains a row and section property.

indexPath.section  
indexPath.row

Here's documentation link for the NSIndexPath class.