We act the same, so its hard.
ESFP's can be flaky and unobservant and miss things, which make us look very un S-like. (I'm not sure why we are this way, I used to think I was an N for this reason.)
ENFP's can be the same way.
We both may appear to be boundlessly happy and silly all the time. But there is a very fundamental difference in our thinking. Since you can't read minds, its hard to tell.
Usually I can tell because ESFP's will talk about facts and details in their stories, and use no imagination
ENFP's are more likely to trail off into "wouldn't it have been funny if I said this" or "I wonder what would happen if," or "I can picture so-and-so doing this." and its actually not facts at all, really, its theory, (even if its not always theory in the intellectual sense.) Also, I think ENFP's may be more likely to be caught in the act of exaggerrating something that happened or change it around a bit, or miss important details in general.
Its definitely not in the actions, anyway, its all in the conversation.
I wish I could think of a good example but I can't right now.

I'm at a loss for example.