Ring power adapter plugin 2nd Gen + & - identification

On wiring up a Ring external siren I used the transformer plugin 2nd Gen, when i turned it on it didn't work. I called Ring Support and they suggested I swap the wires round. I said they are identical so no way of knowing which is positive or negative? When I swapped them it worked. How as an installer am I supposed to know which is positive and negative? Currently it's a 50/50 chance it will work. Any insights on this?