I dont know about 5120SI but in 5120EI jumboframes are enabled by default.
This means that when you do "display current-configuration" it wont show anything regarding jumboframes because its default. Only interfaces with jumboframing disabled will show up with "undo jumboframe enable" in the configuration.
In order to verify if your interface allows jumbos or not you can do:
display interface GigabitEthernet 1/0/1
Which will have a line that informs you of current status:
Forbid jumbo frame to pass
In the above case jumbos are disabled.