[robhack]

Well, for one, you could not know the tunes and think it's all random chromatic scale piano playing and thus all played « correctly ».

[bananaboy]

If you have some musical knowledge or experience it's pretty easy to tell which tunes are wrong even if you've never heard them. Assuming you have knowledge of the western musical tradition, e.g. modes, major/minor keys, etc, it's usually pretty clear where a melody should be going even if you haven't heard it (i.e. it will probably remain in the tonal centre of the piece).

[robhack]

That's interesting, do you have any links you could point me to in order to learn more about this? For instance, let's say we're in C Major.

Tune1 = C D F A G C

Tune2 = C F D A G C

How can you tell which (if any) is wrong or not and how?

[rawTruthHurts]

If you don't recognize the songs you don't know if something is intentional or not. You can stay in-tune while not playing a song correctly.

[Lewton]

Except it's made clear from the way the test is designed that that's not the case