It's not a long press, but either a slight upward flick, or tapping the eclipses (...) at the right hand edge - this will bring up the textual labels for the round buttons in the app bar.
Rowland is right, there were no such feature as long press (only in case if you are talking about long press on list items: messages, pictures, etc). I think, you mean by that
Application Bar buttons. For example, if you want to know what buttons do in specific application, you should tap on
.... For example, Speed dial (Phone app) has
Phone book and
Add application bar buttons.