How would somebody know if they've done any damage?
It takes at least 12 hours before we can tell if anything has happened. The thing is, if people just saw the sun briefly without a protective filter — just a fraction of a second — the chances they've hurt themselves are very low.
If they looked at the display of a camera or a smartphone but didn't look through the optics at the sun, they're in no danger. While it may look bright, it isn't all that bright by comparison. They were not getting direct optical radiation from the sun in that case. It's just a duplicate of whatever the sensors see, and there's no danger in that.
If, for some reason, they forgot to use their filters entirely — well, [permanent damage is] always a possibility, but I would hope that after all the publicity, that that didn't happen.
What might someone's symptoms be if they didn't use any filters and looked at the sun?
It wouldn't be until 12 hours later that they would even notice their vision was changing. It might not be until [the next] morning when they wake up and realize that they can't see that well.
[The symptoms would be] blurred vision, where the very center of the vision might have a spot, or multiple spots, that were missing in their vision — that were very, very blurred. Around it, there might be some clear spots. It really depends on exactly what happened, and what kind of injury there is at the back of the eye. [As Chou told us earlier, this kind of vision loss can get better over several months to a year. But about half the time, it's permanent.]
What should someone do if they're worried there could be damage? Should they stay indoors or wear sunglasses?
Seeing an optometrist is faster than getting to see an ophthalmologist. If there is damage, the optometrist would refer the individual to the ophthalmologist for further assessment and management in any case.
The damage, if any, would not be mitigated by staying indoors or wearing sunglasses, but these are ways to make vision more comfortable as the individual recovers.
I used an eclipse viewer that came from a reputable seller. But even after looking at the sun for a few seconds through that, I felt like my eyes felt weird and needed time to adjust.
The thing is, if you look for a sustained period through the filter, you're stimulating only one set of color-sensitive receptors at the back of the eye. So they'll get more work than, say, the receptors that are responsible for green and blue light. And the result will be that for a brief time, you'll suffer the same kind of after-image effects that you get from looking at a very bright headlight or flash from a camera.
You'll recall how you'll have spots in your vision that are sort of the opposite effect of what you normally see. That kind of thing will happen if you look at the sun for an extended period of time. It'll feel a bit weird as your eyes recover from that. It doesn't mean you've hurt yourself. It's just that you've overloaded some of the photo receptors, and it takes a while to get back to full function. In most cases, it's going to be just a few minutes. There's no lasting damage from that. You would have to be staring at an extremely bright image for minutes at a time before you started to get any clinically significant damage.
I watched it through my eclipse glasses but not for a sustained time. I put the glasses on, took a glimpse, looked away, and took them off. I didn’t see the point in staring at it the whole time. It took about 3 hours for it to move across the sun. I think I took a quick look about five times total. The exciting part for me was watching it get darker for about 20 minutes, go totally dark, and start getting light. It confused the heck out of the night bugs like jar flies. :-)