From a Metadata Server perspective, the primary node is known as the "master," and it can change in the case of failover. You can determine the master node by following the instructions under Monitoring Clustered Metadata Servers at http://support.sas.com/documentation/cdl/en/bisag/68240/HTML/default/viewer.htm#n1qmrerr6rvigxn132cnapd3r2c1.htm.
From a Middle Tier perspective, if you've forgotten which machine is your primary node, there are certain Web apps which are not clusterable, and therefore only live on the primary node. You can find the list of such apps in the Exceptions to the Middle-Tier Cluster Support section of http://support.sas.com/documentation/cdl/en/whatsdiff/66129/HTML/default/viewer.htm#n1roydrale7528n14alj43gi257f.htm.
From a SAS Application Server (compute tier) perspective (I'm assuming you're not talking about SAS Grid Manager), there's not really a primary node. You just have peer machines which host multiple servers (workspace, stored process, pooled workspace, etc.).
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