How can I tell if my iPhone is charging when it's dead?

How can I tell if my iPhone is charging when the battery is dead?

Sometimes, when it's dead, when I plug it in to charge, I see a battery icon with a red line and a white cable icon. I don't know what that means. Does it mean "plug in your phone to charge it"? Or does it mean "your phone is plugged in"? iOS screen is black or nearly black when dead, so I can't see anything.

Because iPhone (iPhone 7+) is so slow to charge, it takes a long time to charge it enough to turn it on. But sometimes I think it's charging but it never powers on, so I end up waiting for a while, then switching cables or plugging in to an alternate source.

On Android, the battery looks like it's filling up when it's is charging from dead. That's what I want to see. I've owned iOS devices for four years but could never figure this out.