You can Lock/Unlock Transaction codes via SM01.
To View Locked Transactions you need to look in field CINFO, table TSTC.
Goto transaction SE11 or SE16 and view the table TSTC, the field is CINFO. Enter "A0" as the "HEX01 data element for SYST" starting value and "A9" as the ending value.This will list all the transactions locked in the system.
Locked transactions can be viewed using report RSAUDITC executed using SA382
Instead of doing such cumbersome viewing of table, you can run ABAP report rsauditc to view locked transactions.
Note: The CINFO field description is “HEX01 data element for SYST”.