How to differentiate "imposter syndrome" from actual lack of suitability for grad. school/academia?
We can’t tell from what you write how promising you really are.  But I can say you’re showing many classic signs of impostor syndrome.
Believe the assessments you receive from instructors.
At least in my experience (the US and NW Europe), “sarcasm” is very rare, and considered quite unprofessional.  I would certainly be shocked to see it used in a letter of recommendation, or a formal course report, or anything like that.  It might be used in informal personal feedback, but this would be unusual, and considered by most academics today as inappropriate and unprofessional, or (at best) regrettably old-fashioned.
Grade inflation is a realistic concern to some extent, as is “praise inflation” in letters of recommendation.  But it sounds like you’re taking this concern far over the top — don’t discount grades or praise entirely.  If your instructor praised the paper, it really does mean he was impressed by it.  Sure, you are aware of the flaws in it — but he saw those flaws, and nonetheless remembered it as very impressive overall.  (Incidentally, it isn’t at all unusual for a paper to have some sloppiness, but still be extremely impressive by showing good insights, knowledge, or exposition.)  Which brings me to:
Don’t judge yourself by a double standard compared to your peers.
There’s a saying: “We compare our own behind-the-scenes to everybody else’s highlight reels.”  Everything you write sounds like what that describes: When you get praise or positive feedback, you are looking for reasons to discount it; but you are taking your peers’ positive feedback at face value.  At the same time, you say you see lack of confidence in you from professors, but everything concrete you mention is positive.
These are all massive red flags for impostor syndrome.  It’s always hard to judge our own capabilities, but when in doubt, look at the most concrete feedback you’ve received from people whose judgement you trust, and take it at face value, especially compared to the feedback they give your peers.  If you look for specific reasons to discount it, you’ll always find some, because you know your own life and flaws better than anyone else.  (Just like when an arrogant person looks for reasons to discount criticism, they can always find some good excuse.)  So, fight any tendencies to discount feedback, either positive or negative.  Try to believe your professors’ assessments and feedback, praise and criticism, and take both on board.  Good luck!