How to check if a record is locked or not in Apex during an approval process?

I believe it's possible to query for the existence of an Approval Request related to your record, I believe this is via: ProcessInstance. Try checking out:https://www.salesforce.com/us/developer/docs/apexcode/index_Left.htm#CSHID=apex_process_example.htm|StartTopic=Content%2Fapex_process_example.htm|SkinName=webhelp

Additionally, you could write a method to attempt to update the record, and trap the exception message that comes from the record being locked and hide the buttons based on that result!