One can see the build version in the ‘Manage BPEL Domain’ section of the ‘BPEL Control’ web app (AKA BPELConsole). In the above screen snippet the version is 10.1.3.3.0.
This information is also available on the top line of the log files in $ORACLE_HOME/bpel/system/logs so you can still get the version info if the BPELConsole is inaccessible.