"Experts" generally claim to be able to distinguish between "things that might look or seem similar but which are not". And/or, people believe they can.
The classic example is probably the ability to distinguish between, say, 2 wines; more generally, between Recipe X cooked well, and Recipe X cooked not so well.
But do you know how to measure that claim?
Suppose your friend says "I can distinguish between Coke and Pepsi". How would you show that that claim is true, or false?