Real easy to tell visually. Right behind the trigger is supposed to be a little tab. If there is just an oval hole behind the trigger and no tab then you know right away it has been removed. Most people simply remove the safety lever. However, a few grind them to make them release "easier." To test for that open the lever far enough so it is not touching the tab and pull the trigger. If you can't pull the trigger everything is working correctly. If you open the lever all the way and look at the base of the trigger you can see the end of the safety blocking the trigger. Push the little tab with your finger and you will see how it works. What's kind of funny is that people get all excited about removing the interlock from a 73 and don't seem at all concerned that the 66 and Henry don't have one in the first place. The interlock is an "improvement" on the 73 and I leave them in. The .44-40 round had more power than the .44 Henry Flat, so an out of battery discharge (OBD) could get exciting. The early 73's didn't have the interlock. I have an original without the interlock. They were added after several years of field use showed there was a potential OBD problem. The reason most people remove them is because the stock interlock spring is STIFF and makes closing the lever difficult in some cases. Hard to grind that spring and if you bend it it to far it oftentimes just leaves the interlock flopping around inside the gun. Now there are aftermarket coil springs that make the interlock easy to depress, but keep it fully functional. I leave it in because sometimes at high speeds I would pull the trigger a micro-second to soon, the round would not fire, and I would eject a live round. Leaving the interlock in helps prevent that. And, of course, it helps prevent an OBD, which was its intended purpose.