The thing is, I don't think that INFJs are particularly 'pedantic', nor are ENFPs 'reckless', so maybe that's your answer?
But let's see...
Any type can pretty successfully come off as another type temporarily, in the right circumstances, so the first thing I would do is try and observe your subject on more than one occasion. Bad moods/downers don't last
that long, so as well as they might be camoflagued, their true self is gonna peek out sooner or later.
But specifically...
Firstly, they use opposite cognitive functions: ENFPs use Ne and Fi, INFJs use Ni and Fe.
Definitions are
here.
For example, as attentive and engaged as an ENFP may be when speaking to you directly, they are quite bad at following up. Unless you're very close (and in constant contact), something like a christmas card is probably too much to expect.

The Fe of the INFJ, on the other hand, is much better at following up and keeping a tab of all that - if it appears like you're close to them, then you are, and you'll know it.
Also a vital difference: if the person is subjected to a sudden, major and entirely spontaneous change-of-plans, an ENFP will rarely miss a beat, but an INFJ may be thrown out of whack and really struggle to catch up.
Does that help?