And quite honestly, I’d far rather elicit a love me/hate me initial response when you first meet me, if that really is the case with only’s, than ambivalence or “she’s ok I suppose, a bit meh”.
I’m assuming this is directed at me. You perhaps missed my earlier, longer post where I explained that I’m an only child myself. So this addendum was intended as self-deprecating. What I’m recognising in these other people is myself! For either good or ill, hence the love/hate.
As I said, it’s not always the same trait or set of traits which gives it away to me. It could be confidence. It could be stubbornness. It could be the way they react to a crisis, in a good or a bad way. It could be a tiny thing, like them asking where ‘their’ pen/cup/chair/whatever is. It’s not a criticism, other than of myself! 
FWIW, these people are rarely my closest or enduring friends. For friendships, I’m drawn to people quite unlike myself, because they balance me out, I suppose. The other only children tend to be the people I either butt heads with, or else fall passionately, ill-advisedly in love with! It’s not a good thing and I wish I didn’t have this internal radar because it’s rarely served me well, but it’s been like this all my life and I’m over 50, so I don’t think I’m imagining it.