What are some obvious cues to tell ENFP and ENTP apart?
In addition to what's already been said, there are Fi (ENFP) vs Ti (ENTP) clues. ENTPs form opinions by thinking through what looks logical according to the information they've gathered, and might not offer an opinion on something if they don't feel informed enough to have a logical opinion. Although some might use Ne to fill in the blanks and not worry so much about informing Ti.
My impression is that ENFPs are likely to be more opinionated about things that touch on their values, and might get angry if you disagree with them on something that violates their values. That is Fi. Everyone does this, but it seems ENFPs do it more than ENTPs.
They'll also challenge you differently in an argument. An ENFP is more likely to see a differing opinion as being something only an asshole would think, while an ENTP is more likely to see differing opinions as illogical or stupid. But the more mature of both types might converge to a more live and let live attitude.
Edit: Also, I think both types like to argue to learn more, but both types (especially younger ones) may also try to "win". An ENTP will think they won if they see their own arguments as more logical, and an ENFP might think they won if they had more to say and were more passionate about it, or if they've dismissed the other as an asshole.
Both types tend to be very playful, but I think ENFPs are more playful, and usually more sensitive to other people's feelings, especially in ways that they can relate to.
(ENFPs, feel free to correct me if I've misinterpreted you.

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