There are symptoms that flag somebody as a potential 7 to me, for sure. It's the nature of the beast underneath that seals the deal, as I think (like 3s) we often come across as smooth and entertaining on the outside while not actually being so chill on the inside.
I notice things like difficulty sitting still, the relative absence of (outwardly demonstrated) sadness/negativity, charisma combined with nervous idiosyncrasies (biting your lips, laughing during moments of discomfort), and a fairly widespread assortment of interests and skills. If 7s have an occupation they really enjoy, they're often very good at it and might stand out from their peers (just watch out for confusion with type 3).
I recently met a guy at a brewpub who strikes me as possible 7. He initially approached me by playing wingman for his friend, but we had more in common and kinda hit it off. There was something pretty effortless to him, just fun and excited, full of stories, questions, and alcohol. A little bit cocky, but totally able to handle it whenever I'd give him shit.
Anyway, we ended up hanging out for a little bit and I told him I've got a boyfriend. He was really good-natured about it, though he'd clearly become sexually interested.
Eventually we grazed on a few deep topics, and he sort of spontaneously let slip that he was actually trying to forget about someone he'd been in love with for four years. It was very brief, but for half a second all of his rough edges bled through and I caught water shining in his eyes. I tried to seem encouraging and relaxed, but within moments he'd cracked a joke, shrugged it off, and kept right on keepin' on.
I thought, ah, there it is.