I was really intrigued by this topic and found some videos on it. I was skeptical about all the "findings" they had, but as soon as I thought about it more and observed the people around me, I began to see these exact signs!
These pictures were from the videos. It says that Fi users have smiles that seem to move towards the nose vs. Fe users which are more towards the ears. Now, I have noticed this in the Fi-users that I know! I think some people have mentioned that Fi users seem to smile with their eyes, like their eyes get small and I think it may be due to this habit where the smile concentrates towards the nose. I even notice that Fi users' noses (well atleast the dom and auxilary users...) seems to crinkle more when they smile versus Fe users. Now, it is still a learning process for me and I don't have definitive proof yet that this is the case for everyone, but I have seen this pattern and it seems plausible to me.
I actually really like Fi-smiles. I really do think it shows off a more child-like innocence and joy. When I see an Fi-user laugh at something, it is contagious, and I laugh instantly.
Here is more information about it:
Also here is a video to see more of how it looks (it is easier to spot in videos than pictures...but it is subtle and can still be easy to miss):
(go to 1:52min)
(The pictures from above come from the same line of vids, but it is the SeFi version.)
Now some warning, it gets kind of annoying how the captions flash so fast in the vids that you can't read it, but when you pause it at those moments (like the vids say for you to do), then you start learning the technique and things to look for. I don't know if I believe everything on the vids yet...but the whole Fe vs Fi smile thing really sold me, cuz I have seen it in real life before...