it's a lot about how they deal with new things or ideas, or unexpected situations. Other comments are talking about how changing plans unexpectedly my upset a J while a P is gonna go with it. But I think a simpler way is to pay attention during a conversation, Js can't help but make value judgement (hence, J), you'll hear a lot of "that's bad" "that's better than that" etc., and a conversation with a J tends to stay on topic. On the other hand, Ps are all about what's out there, what's new, what's possible. You'll see genuine excitement to been presented with something they hadn't heard or known about before, and are generally more curious and receptive individuals, so you'll get more questions and "that's interesting" "what else" etc., and the conversation tends to take more turns and twists and end up in places that leave you wondering where how you started. That's not to say that Ps never make value judgments or that Js are not interested in new possibilities, but their first reaction tends to something new tends to give it away.