An alternative technique called “divergence” [1] (pulling your eyes apart) is significantly less straining on your eyes than crossing them (“convergence” [2]) while being equally as effective spotting differences, even on video. It’s also what your eyes naturally do when you watch stereoscopic 3D with tinted glasses—the stereoscopic images are pulled out (divergence) not pushed in (convergence/cross-eyed). I’ve been doing this since I childhood. If you get good at it, you can watch side-by-side 3D videos in 3D with just your naked eye (e.g. VR)! I believe there’s a reddit covering the more prurient variety of that.
This is what I do, the only issue is that I don't have nearly as much "range" with divergence as I do with convergence, so I have to make the pictures as small as possible when using it to line up two images (as opposed to autostereograms, which usually have a much smaller divergence offset).
It's a little more abstract since you don't have handy moving-reference-object like your finger, but: Place the picture in front of something deep, like a long hallway. Look off at something in the distance behind the picture, like the end of the hallway. Notice how the edge of the picture is a double image. Focus on gradually resolving the edge of the picture down from double-image to single-image, and then do the reverse by looking down the hallway again and seeing the picture go back into double-vision. Just keep practicing that until you get the feel for controlling your depth perception and then try holding the same depth of the hallway while you turn your gaze to the picture and try the same action with your eyes.
Damn! After reading this I was surprised by the fact that this sounded very familiar.
I actually "practiced" a lot like this because I was always amused to notice how we could basically "see through" objects with this double-image thingy (see experiment below).
So I decided to film myself... and I was actually already doing a divergence! Not convergence!
Thanks a lot for your comment which made me realize that.
Experiment:
1. place your phone (handy size/shape for the experiment) in front of one eye (X), at about 20cm.
2. close the other eye (Y) and look at your phone
3. Open Y and look straight without focusing on the phone. By blinking Y, the double-image should appear/disappear, as if it was unveiling what's behind your phone.
4. By closing X and with Y open, looking at your phone, you should see it displaced from where it was when X was open and Y was closed. The size of this displacement is equal to the size of the double-image transparent part.
[1] https://en.wikipedia.org/wiki/Vergence#Divergence
[2] https://en.wikipedia.org/wiki/Vergence#Convergence