Oh I can tell, it's principally in the aroma and the presence of a bitter aftertaste. But it pretty much stops at the I prefer 100% Arabica over a blend of Arabica/Robusta but a decent blend can pass muster level. It's simply easier to know that any bag of 100% Arabica beans I buy will be good but I'll never know with a blend until after I've bought it.

A cheap blend with a high percentage of Robusta beans will get the old squint eye and I'll politely decline any offer of instant coffee. I recently bought a 1.2kg bag of Lidl coffee beans for £8, 100% Arabica, tried it and it tasted good so I promptly went back and bought another five bags, with the stock I already have that's me sorted until the end of next year.

If you get a coffee and drown it with an overabundance of milk, sugar, caramel, etc, etc it could be any almost any type of coffee and you can't tell the difference. But since people seem to like those sweetened monstrosities {shrug}.