This article explains how to determine whether SRX is using Dual Root Partition.

Starting in Junos OS Release 12.1X45, single root partitioning is not supported on SRX Series devices.

You can also verify this from the CLI.  The following output will appear if the system is running as dual root partition:

root@srx> show system snapshot media internal
Information for snapshot on       internal (/dev/da0s1a) (backup)
Creation date: Aug 3 08:49:17 2020
JUNOS version on snapshot:
  junos  : 15.1X49-D200.3-domestic
Information for snapshot on       internal (/dev/da0s2a) (primary) 
Creation date: Oct 1 12:09:08 2020
JUNOS version on snapshot:
  junos  : 18.4R3-S4.2

For more details, refer to  Dual-Root Partitioning Scheme on SRX Series Devices

2020-10-04: Updated per latest device and JUNOS. Added reference document for clarity.