Only 2 ways to test a battery that I know of other then just using it. Meter test for voltage will indicate if all of the cells are operational and battery hydrometer test will do the same thing, but motorcycle batteries are so small it can be difficult to draw up sufficient fluid to hydrometer test with an auto type tester. Visually examine the fluid in the lowest part of the battery (assuming it has a clear plastic case) and if there is a considerable accumulation of solid sulphide material in there it will short out the plates. If the plates are warped from excess heat that will also short out the plates. Charging rate has a lot to do with how fast the battery will charge and charging at too high of a rate is more likely to warp the plates.
Was the battery stored at room temp or sub-zero? I've rarely had a motorcycle battery last more then 4 years when it has been stored in sub-zero temps, ymmv.
Sounds to me it was just too low on charge to turn the starter motor which puts the highest demand on the battery. Surprising enough the horn actually uses a lot of power, sometimes there is enough charge to lite the lights and not beep the horn.
A meter continuity test is the best way to test your battery ground connections between the motor/frame & - battery terminal, there should be little to no measurable resistance between any of those connections.
... hope this helps some.