Easy to tell in real life once you get to know them- though on the surface they may appear similar
ENFPs usually have something like sadness behind thier laughing though
Once you get to know them though they're quite different- an ENFP wants to know all of your flaws because they find them fascinating- they want to know your psychological makeup in a more sympathetic manner and such. ENTPs just want to know about you in a more coldly clinical manner- find the interesting part then move on and scan someone else's brain. There's a different feel to talking to an ENFP- more depth in a way, less calculations.
Of course I have a good ENFP friend and have tried to pinpoint the differences between the both of us- it's hard to describe NFness though since it's kind of odd and nebulous and such