Heh. A few years ago I got a new manager at work. So I was standing around with everybody else at the conveyor line where we unload product from the truck, and he makes the rounds introducing himself - though I don't notice till he gets to me. So he goes "Hey there, I'm X, nice to meet you!" and I go "Cool, nice to meet you too!" and he walks off. Ten seconds later I'm like "Oh shit! I forgot to introduce myself!" and run after him while the guys around me laugh "yeah that's why he looked so confused". (I didn't even notice he was confused).
Also that exact same thing in the comic happens to me constantly...it's hard to remember when you're supposed to make socially-appropriate gestures, and exactly what gesture to pair with a particular gesture.
With enough self-training and perception, one can pick up on...well, at least some subtle cues. As Brun stated, she was able to read what was going on with Clinton at the time, since he made enough outward signs and speech implications for her to parse through. But even a little masking can make it far harder to figure out.
And she also picked up on Clinton's subtle condescension in panel 3. I think she also picked up that it wasn't intentional; he was trying to be nice - so she firmly but gently corrected him. If he was being intentionally condescending I'd expect a response more along the line of "I see. So you think I'm stupid." and disengage. As for Clinton, he's trying to be nice, but doing it in a way that NT people stereotypically do - he's essentially saying "Nah, you aren't broken, remember that one time you did the thing just fine? You're really fine as anybody else." This both denies aspects of herself that she KNOWS about herself, difficulties that she KNOWS she has, and implies that having them makes her less-than; that the standard - the "normalcy" - is neurotypical.
Which is something that I think the autistic community pushes back on - you can understand that as "differently abled" rather than "disabled". All that said...sigh...it's nice that this is a comic, because in real life, even if a person isn't explicitly bigoted against autistic people...well, there's a lot of subtle social cues, from gestures, facial expressions, tone of voice, to even the information-light "conversation about the conversation itself" way they speak that...if you're just not any good at hitting those notes, people will read you as "off", or "retarded", or just not someone they really have any interest in bothering with. Humans evolved to be social animals, after all - you gotta do the social thing right to really be included in anything.