How do I know if my wood front door's finish is weather proof?
All exterior wood finishes that are "suitable for sun exposure" still slowly degrade in the sun (it's worse than water, actually) and need to be renewed - Just ask the folks with wooden boats. So whether or not the current finish is "weather resistant" (since "weather proof" is not really accurate) it will need to be renewed. 

The simplest approach is to re-coat it now with spar varnish or some other exterior clear finish (spar urethane would be the other common one) and then you know what it's got on there. Then plan to re-coat before significant damage accrues - it's easier to maintain a finish than to try and restore the look of wood that has been damaged or discolored by weathering.