You can tell this by pressing Menu 6-1-3( on non Hopper/922 receivers). If you have 119, 110, 129 you will be on Western Arc. If you have 61.5, 72.2, 77 you are on Eastern Arc.
If you have a Hopper/922 it would be Menu-Settings-Diagnostics-System Info.
Or you can tell by looking at the dish. Western Arc has three equally (or close) spaced LNB heads. Eastern Arc has two together and one further spaced.
Or you can tell by looking at the dish. Western Arc has three equally (or close) spaced LNB heads. Eastern Arc has two together and one further spaced.
He/she asked "which arc my dish is". Pretty sure that indicates they already have Dish.
Easy check, depending on the receiver of course, is to simply hit "Info" on the remote two times. It will tell what satellite the channel being watched is coming from.