Obviously when you look through binoculars and you see a double image, it clearly means they are out of collimation (both barrels not aligned with each other).
But in some occasions the error is very slight, and your eyes compensate to correct this. This correction is obvious when you move the binoculars away from your eyes. Your eyes then “switch back” to normal, and causes a strain.
A good way of checking for slight misalignment is that you look at a target through the binoculars. Then with the binoculars still held up to your face, close your eyes for 5 seconds to let them re adjust. When you open your eyes, do you see a single image straight away. Or do you see a double image that slowly corrects itself. If you get the later, then the binoculars are out of collimation.