How can I know my brake fluid needs changing?
The suggested interval to bleed your brakes seems to different among manufacturers.
While  SRAMs techdocs explicitly mention to replace the fluid every year, Shimanos techdocs (p20) does only recommends to replace the oil if it becomes severely discolored. For recreational use, I think you're safe bleeding once every two years but depending of the usage and storage it might be quicker.

Magura on the other hand claims their Royal Blood oil (p21) does not age. I've ridden an MTB having HS33 brakes with 10 year old oil and indeed had a firm braking experience. 

Are there any changes I can notice in my brakes that will let me know its time to change the fluid?
Replacing the fluid before the regular maintenance can be required if air got in the system. Magura suggests to replacing the oil if:


the brake does not respons immediatly when the brake lever is actuated
pressure point is not clearly defined, it is spongy or does not remain constant
the brake hose has been changed


If you have multiple bikes and like to do some own maintenance, it's even worth looking into buying your own bleeding kit. Shimano has nice technical documentation and there are various videos online giving you a feeling how to bleed your specific brake type.

Small sidenote, regarding storing your bike: ideal you put the flat and upright, so there is less chance for air coming in the brake system. While transporting the bike be sure not to press the brake levers without having the wheels in your bike, as it can cause the pistons coming out too far.