There are a few different chassis that have the master cylinder in a good place for the brake pedal but a lousy one to service or check the fluid. You could use a bulb pump like this to fill the reservoir:
Siphon Pump - Save on this Fluid Siphon Pump
Try to see the level with a small mirror:
It's not ideal, but you don't have to do it very often.
I've also seen it done with a long, thin hose (think large aquarium hose) a funnel, and poured in from outside the body above the level of the MC reservoir. Problem with this method, it takes two people, one to hold the funnel and pour the brake fluid, the second person underneath watching in the mirror. The person underneath also must pay attention to the fill level, you have all the fluid in the hose to account for.