Hi
you would have to know what cards every poker player was holding,
to know who has the best hand.
and, unless you've had a peek or they've mentioned what they've purchased or if you follow premiums to see who catches and buys what...
then you might get an understanding of what kind of cards they prefer to draw.
on other side, you could look at what they sell and for how much and maybe even for how long they held that card, before turning it over.
these are things that give insight if you're bright and can connect dots mentally, as side notes.
still, "most impressive", depends on who is looking at what and how they compare their cards, to the other players cards.
imo...