Warning--DX door locks

[Wizard52]

To clarify, this has nothing to do with the door key operated lock, it is the latch that holds the door shut and operated by the door handle?

How can you easily test it?

Spot on. Can't think of any test but in my case I had some warning as it was difficult to lock and unlock so I had a look and so lucky that door was open when it eventually could not be unlocked when the "latch" was in closed position. There is a small sliding bit under the latch which would not move to allow it to unlock. I could move it with a small screw driver while door was open but obviously it would be impossible when door was closed. No amount of juggling the connecting rods from door latches helped so I presume something internal in mechanism broke.