On Colt Ar15's how do you tell that there is a sear block in place. I hate to admit my ignorance but there it is. I'm in the market for something Politically incorrect and I need to be able to distinguish between blocked and unblocked receivers. Any description would be appreciated but a picture would be even better.

Look at your lower receiver and you might see a piece of metal that seems to serve no functional purpose.

There might be a rectangular piece of metal that resides where the auto sear would be.  Maybe about 1/2 an inch long.

It is pinned in from the right side of the receiver.

It may say Colt on it.

Look behind the hammer, in front of the rear push pin.