SharePoint 2013 Build number

4/26/2014 - Service Pack 1 has been rereleased. Please read this blog post to see how to tell the two SP1s apart

To see which build your farm is, go to Central Administration > System Settings > Manage servers in your farm (/_admin/FarmServers.aspx)

Or using Windows PowerShell: (get-spfarm).buildversion

To see which build your farm is at with Fiddler, open the site in question and view Inspectors > Headers and under Miscellaneous check the build under MicrosoftSharePointTeamServices header.

To see which build your products are, go to Central Administration > Upgrade and Migration > Check Product and patch installation status. (/_admin/PatchStatus.aspx)

To greatly reduce the time it takes to patch a SharePoint 2013 server, read this blog post by Russ Maxwell, Why SharePoint 2013 Cumulative Update takes 5 hours to install? It explains why the process takes so long and provides a handy script that makes the patch install much more quickly.

Do yourself a favor and check it out.

AppFabric needs to be patched too.

Patch terminology KB article.

SharePoint 2013

Office Web App
2013

To see which version of the OWA (also called the "Web Access Components" or "WAC") use the following PowerShell:  

(Invoke-WebRequest
https://owa.contoso.com/m/met/participant.svc/jsonAnonymous/BroadcastPing).Headers["X-OfficeVersion"]

Workflow Manager 2013

To see which version of Workflow Manager and Service Bus are installed, use the following PowerShell:

(Get-ChildItem -Path "C:\Program Files\Workflow
Manager\1.0\Workflow\Artifacts\Microsoft.Workflow.Service.dll").VersionInfo

(Get-ChildItem -Path "C:\Program Files\Service
Bus\1.0\Microsoft.ServiceBus.dll").VersionInfo

or

(Get-ChildItem -Path "C:\Program
Files\Service
Bus\1.1\Microsoft.ServiceBus.dll").VersionInfo

When you install Workflow Manager CU 1 you also have to install Service Bus CU1. Also make sure you patch the Workflow Manager client on SharePoint servers that are not running the Workflow Manager service. See more details on how to patch Workflow Manager on Andrew Connell's blog.