I went through this process 2 years ago with a 3" 624. To reiterate what Keith Brown and Hondo44 have reported there are no markings on the gun to indicate whether or not it was inspected. I sent my 3" 624 back to S&W and received it back with a letter that gave it a clean bill of health.
I also bought a 4" 624 that had the red letter "C" on the end label. Inside the box was a letter sent to original purchasers advising them of the recall.
Here is a link to the thread that discusses the issue and has a copy of the original recall letter.
http://smith-wessonforum.com/s-w-revolvers-1980-present/171766-624-629-recall-notice.html
I also bought a 4" 624 that had the red letter "C" on the end label. Inside the box was a letter sent to original purchasers advising them of the recall.
Here is a link to the thread that discusses the issue and has a copy of the original recall letter.
http://smith-wessonforum.com/s-w-revolvers-1980-present/171766-624-629-recall-notice.html