These two signals, the Se-Ni vs Ne-Si eyes and the Te-Fi vs Fe-Ti smiles, have proven the hardest for me to see. Especially the eyes. I've given up! I now rely on finding them by elimination, ruling out what cannot be based on some other signals or reasons.
“If every tiny flower wanted to be a rose, spring would lose its loveliness.”
A more detailed description of Fi and Fe facial tension:
Aqua - I hear you! It can be really challenging to differentiate between cues when you're starting out, I still have trouble with Fi vs Fe smiles sometimes. From what I've seen, you have a pretty high level of accuracy when identifying people's types though, so don't be too hard on yourself!
(I made these graphics partly with what you said in mind.. I hope they can be of help to you. Please let me know if you think they could be improved upon to make better learning tools.)