I found some unmarked cables in a box, in my storage area. I can't tell if they are brake or shifter cables. Is there a way to tell visually?

Thanks.

I assume this means they have barrel ends?

Yes. Barrel ends, they have.

Having said that, I've seen brake cables ends that look similar to shifter cables ends, and vice versa.

Generally on brake cables the cable is thicker than shift cables. Roger

Yes, Brake cables are usually 1.6-1.8mm thick and the barrel ends connect to the cable at the middle of the circumference. Shift cables are usually 1.1-1.2mm thick (although classic pre index ones can be thicker) and have the cable connecting at the flat end of the barrel. Exceptions do exist but this covers most all current ones. Andy.

Besides the width, the type of head determines the function because it has to match the appropriate lever. Gar wires have smaller heads in the 4-5mm size range. OTOH brake wires have stepped "egg" shaped heads or a squat barrel shaped head with the wire exiting the side. Both brake cable heads are in the 8mm size range.

Here's a photo which may help. ABC are brake cables, CD gear.