First post. I'm still reading the iOS Human Interface Guidelines and haven't come across this scenario, yet. When displaying multiple columns in a table, what is the appropriate indicator to the user that they can change the column used for sorting?
Well, the UITableView is a one-column table. So, you can usually not change the sorting. I'd recommend reading the Table View Programming Guide. It says right at the beginning:
A table view has only one column and allows vertical scrolling only. It consists of rows in sections. [...]