I just played with my vision a bit and came up with a way to self-test our eyes. Haven't yet seen this method anywhere so I thought I'll share the discovery here. It's a test for alignment of the eyes and it's really simple. All you need is some distant (at least a few meters away), stationary and distinct point, for example a peephole/knob on door or a distant car parked on the street. I liked doing it with a red light of a construction crane from site nearby at evening.
To start, look at that point far away from you. Keeping the focus on that point, close one eye. Now slowly open second eye (do it really slowly to see in which direction things are moving). If there is any kind deviation, you should see a second point appearing on the image from the eye that is being opened, which should move towards that first point, and eventually merging with it, so that there is now one image (of course points in the peripheral vision are also doubled and merging). Close that eye again, wait a few seconds (so it relaxes), and slowly open again and notice how images are merging. You can try it with the second eye and see if the initial shift after opening eye has similar magnitude, or if there is also component in other axis present. That's it.
Now what does it mean? Big deal you could say, that merging after opening eyes is normal. That's what I thought to myself as well. Maybe it is normal (I've heard that most people have some degree of phoria, it's just compensated and asymptomatic), but it does tell us how our eyes are aligned.
I have CI (convergence insufficiency), which means exo deviation (exophoria). What I noticed is when I closed my left eye and then slowly opened it, the second image appeared on the right from the first one. Let's visualize what's going on. This is when I look at the far point in front of me with two eyes, I see one, merged image, so it means the eyes are pointing perfetctly straight:
Now if I close my left eye, it no longer needs to fixate on any point in space. So it will relax and because I have exophoria it will drift outwards:
Now when I open it again, at the beginning the left eye should have the point to the right, becuase it drifted to the left and that's exactly what I observed when doing the test. Now the brain wants to have one image, so the eye rotates back so that it points straight - and this is what you see after opening the second eye when images are merging together. And - at least in my case - the same thing happened with the other eye. So that's how I know that both my eyes rotate outwards when I close them (and that's exophoria). However, I also noticed that after closing and opening my right eye there is also a small but apparent vertical shift (so the image in fact moves diagnonally), which is not present on the other side, which is a totally new discovery for me, becuase optometrists did not diagnose me with vertical phoria. I will have to ask for another test on next appointment.
I believe that for esophoria the mechanics is similar, just reveresed. And that's it, maybe it's obvious for some, but for me it was a nice discovery and another confirmation of my exo deviation diagnosis, which is nice. I also recently stumbled here upon a post of a person who wasn't sure if they had exo or eso deviation. I believe they can check that with this method. Of course, this method is not quantitative, it doesn't measure anything, but it gives as a clue how eyes are aligned and after doing it, we are more consciuos of our vision system movements.
UPDATE: I did the test at night. I tried same test after waking up next day and the effect was smaller, definitely more difficult to notice. Looks like during the night our eyes rest and the deviation is different at the end of the day. When it's more difficult to notice, you can also just wait longer until you open other eye (so that it relaxes more) or as an alternative try to close one eye and keep switching closed eye to notice the image shift, preferably with pen/pencil held in front you as a reference point also to make it easier to notice the shift..
UPDATE: What I should also mention this method checks for deviation at distance. However if we have exo at distance, we'll probably have exo at near (the difference will only be in angle of deviation.), not sure if it's same with eso. You can do the same test to test deviation at near. Just look first with closed one eye at a point 40 cm from you, let's say your finger. Wait a bit so the other eye relaxes and drift to its default position and while focusing on a finger, open your second eye. You'll see the finger doubled (if you have a phoria). Notice the position of the doubled finger.
And of course, if my understanding is wrong, please correct me!
PS Later I'll improve images later and make text corrections if necessary