Go to Clients, then in the lower-middle pane hit the "Search Clients" lnik to open up the search window.
From here make sure you have the following query settings chosen:
Find: Computers
In group: Top-level folder (called My Company by default)
Search subgroups checkbox: ticked
Search filed: Client version
Comparison operator: < (less than sign)
Value: 12.1.671
Then hit the search button, and you should be presented with a list of your managed SEP clients that have a version of the software lower than the RTM 12.1 build. Just FYI, the build number for the beta client is 12.1.601.4699, so you can search for an exact match of that as the client version if you want.