The fundamental technique for doing this is to multiply the volume times the density. So the first thing you need to know about the object is its density, before you begin estimating. How you would discover the density of an F-14 is beyond me. This is a plane made of lightweight metals, and other things, and mostly tons of fuel. What always fascinates me about fighter jets is that they weigh so little empty, and so much fuelled. They are basically flying fuel tanks. You can probably get close to the right answer for a fighter jet by just knowing the density of jet fuel!
The Internet may be able to help you. A search using "f-14 tomcat specifications" provides a website which provides the mass of an F-14 Tom Cat: http://www.geocities.com/CapeCanaveral/Hangar/3103/Specifications.html