How do I determine what the current SP/CU level is for my Exchange 2013 server?  My search through google isn't turning up any kind of command.

Thanks.

In the EAC, go to "Servers" and you'll see the versions for all of the Exchange servers you have.  Then go to http://social.technet.microsoft.com/wiki/contents/articles/15776.exchange-server-2013-and-cumulative-updates-cus-build-numbers-en-us.aspx to cross compare your servers version numbers to what's in the TechNet Wiki.

Or in the Exchange Shell, run the following:  Get-ExchangeServer | Select Name, AdminDisplayVersion

Hello

tip:https://gallery.technet.microsoft.com/office/Get-Exchange-20102013-41701618

My server is CU3 level.  Do I need to install each CU (4 through 8), or can I just install 8?

Thanks.

You can go right to CU8.  To be safe, make sure you have a backup of AD before applying the update.  Schema updates will be applied, and if the install doesn't ask you to reboot when it's done, it's a good idea to do so.  Good luck.