"Aia i hea ka pila?"

Translation: Where is the check?

[HklaniClee]

Better to be more specific. A check (like a bill or invoice) is usually pila ʻaiʻē, and a bank check is usually pila kīkoʻo. That avoids the confusion.

[GeraldMath4]

Then how WOULD one say, "Where is the (musical) instrument?"?

[jdmcowan]

Presumably in exactly the same way. But in any particular situation, it should be pretty clear which you mean. The situations where you might mean either, causing confusion, would be very rare and unusual. Of course, since Duolingo does not include context, I would guess that this sentence would accept either translation (and if not, then it should be reported using the report flag).