"Well spotted!" sound very British to me, but I find myself using it all the time as British expressions creep into American speech especially after "Harry Potter". For example, nowadays, (or perhaps just now, today) I often wish people "Happy Christmas!".
"Good catch" feels more American.
When Americans (not sure about British) want to praise someone who has seen something others haven't:
"You've got good eyes!" (or just "Good eyes!")
"Sharp eyes you've got!"
"You've got 20/20 vision there!" (less common, but acceptable)
Other more generic phrases:
"Well done you!" (although this can be perceived as sarcastic)
"Good on yer!" (very, very Australian)
(On getting a question right, or knowing something) (British) "Full marks!" (American) "You get a gold star!"