To answer your fair question with words alone is a challenge. I am uncut and will try to answer it.
That still photos leave you baffled does not surprise me. The problem is that cut and uncut look very very similar when both are hard. Also, when you cut the foreskin of a 1 day old child, it's hard to predict what the outcome will be for the man that baby boy will grow up to be. So circumcised men vary in appearance. The only complete answer is to spend some time carefully studying an example of each kind of penis. Even a video does not fully get across the complexity of an uncut johnson.
Before puberty, uncircumcised boys have long pointed foreskins and look pretty much alike. But what they look like when they are full grown will vary considerably, in particular how much of the head is covered when they are hard or relaxed. You see, puberty makes the penis grow more than its foreskin. This is one of many reasons why cutting a baby's foreskin is not a good idea.
There are cut men who look like uncut men with short foreskins. There are uncut men who spend their adult lives with the head always fully exposed, so that they look cut.
If you can find a ring scar on the erect penis, he's been cut for sure. But the ring scar is not always easy to see. If there is a lot of freedom in the penis skin when you do HJ, he's probably uncut. If there's a frenulum on the underside of the penis, he's most likely uncut.
Some years ago, I used to follow the blog of a woman who was a nude model and sex radical. One day she made a post on the top 5 unresolved sex questions in her mind, questions that she thought high school sex ed should address. And question #1 was, you guessed it, the difference between cut and uncut penis.
When I was a teen, nearly all boys around me did not know what circumcision meant and could not have answered the question "are you circumcised?" There GFs and sisters would have been even more clueless. Many women learn the difference from a college affair with a dude not born in the USA, or helping take care of an uncircumcised baby.