Use the phrase "I discovered that..." to talk about learning something by noticing new details.

This is different from "realizing" something, which also means to start to think something. But "realizing" things takes more mental work than "discovering" things:

As I was riding the train home, I realized that I'd left my keys in the closet at work.

You could also use the word "noticed":

I bought these here earlier today, but when I got them home I noticed that one of them seems to have a hole in it.