On most 'slide arm lever' doors it is impossible from the outside to tell if the girt bar has been fully retracted.
On the 757,767 and 747 the action of pressing the red "press" tab and/or external movement of the door handle (type depenent) disarms both the slide and the EPAS Gas bottle that blows the door open when armed.
As the cabin crew disarm the door they will run through a series of checks "cross checks" and a variety of visual cues will confirm if the door is disarmed or not. If there is any doubt when you knock on expect to see a thumbs down or no reply.
So far as distance to drop is concerned as far as I am aware the 757, 767 and 747 all have a distance of about 15-18" to drop before the pin is pulled and the slide inflated.
The 737 is the exception to all this as to arm the door simply is a matter of latching the girt bar (the bar that attaches the slide to the aircraft) into locking points in the door sill. if forgotten open the exit from the out side will still deploy the slide pack if not the slide as that has pin to pull like the other models.