How did I know?

My brother and sister asked me last night how did I know I was adopted?
Ever since I can remember, I always suspected I was adopted. They remember me asking about it several times as a child and an adult. They said it was so hard to not answer me truthfully when I'd ask but they swore to our mother.
I don't know really. I knew I didn't really look like anyone. My brother was 11 yrs older then me and my sister was 7 yrs older so I wondered by the large gap. My mother would have been 37 and in the 60s, that wasn't all that common. My birth certificate said I was born in Penn. when we lived in Jersey but that was explained away as an afternoon trip when she went into labor. Although, I thought it was funny after 3 kids she wouldn't have recognized the signs of labor and head home since it was only about 2 hours away.
There were times I just felt different. I remember feeling my mother loved my sister more. She seemed prouder of her accomplishments. But a lot of that could be normal sibling stuff. I can't say they really treated me any differently. I just knew...and I don't know how.
I've read some stuff about babies searching for their mother's after birth and if they can't find them it upsets them and they are aware of it. I don't know how much all of that is true but, I did not have any factual reason to suspect, like a suspicious letter or anything. No one slipped up and said anything, ever. No one treated me differently. I just knew, somehow.
I'd like to learn more about that I think so back to researching!