There is a variety of plastic wrap that is advertised as "Microwave safe". Cover the vessel you are heating with this type of wrap so that the edges are sealed but the center is loose enough to contact the fluid being heated. Before ANY vapor can escape the vessel to be ignited, the wrap will have to rise from contact with the fluid and baloon up. This action ought to be easily visible throuh the window. Heating will instantly stop when the microwave is turned off. (No more energy will be supplied to the fluid.) Uneven heating now plays no part in the process. If the wrap swells or even just barely leaves contact with the fluid surface, the vapor pressure is at least equal to atmospheric and the fluid is boiling. Until that point is reached, there is no possibility of the vapor being ignited by spark or open flame.