How to check whether a record has been locked in the Approval Process
Step2: In your Approval Process in the INITIAL SUBMISSION Action , create a Field Update. Update the FLAG to TRUE.
Step3: In your FINAL APPROVAL and REJECTION Actions, create a Field Update. Update the FLAG back to FALSE.
Step4: And done!!!! You can now find whether a record has been locked simply by checking the FLAG value. Be it in your Apex Class or Visualforce Page.