The easiest command to find is cat /etc/release like below:
# cat /etc/release
Solaris 10 5/08 s10x_u5wos_10 X86
Copyright 2008 Sun Microsystems, Inc. All Rights Reserved.
Use is subject to license terms.
Assembled 24 March 2008
Solaris 10 10/09 (Update 8) Patch Bundle applied.
If Above command doesn't work you can also use pkginfo like below:
# pkginfo -l SUNWsolnm | grep VERSION