You can check the cluster status.  The Cluster ID with the status of Master is the primary or serving the traffic.  Also, base on the priority of the cluster ID, the cluser ID with higher value is set to be master or primary.  The status below show that both cluster ID 1 and 2 are master on the device tm2.

tm2#show cluster virtual status
ifname = outside
vcid  on/off vc state
  1    on     master
  2    on     master

Webui:
System Configuration --> Clustering