Use a mass spectrometer to test an element or compound, according to the device's manual.
Look at the relative abundance on the mass spectrometer's output. This is a graphical representation of the number of each type of isotope. The isotopic mass is listed at the bottom of the bar graph and the relative abundance on the left side of the graph. Each isotopic mass represents a separate isotope, and the relative abundance is its quantity. Read over from the top of each isotope's "bar" measurement to the measurement on the relative abundance scale. Write down these numbers.
There may also be a number above each isotope's "bar" measurement. This number is the relative abundance for that isotope.
Add all the relative abundances together. As an example, boron has two stable isotopes, boron-10 and boron-11 -- the number corresponds to its isotopic mass. The relative abundance of each isotope might be 23 and 100, respectively. So the total abundance is 123.
Divide the quantity of an individual isotope by the total abundance, and then multiply by 100 to calculate its percent abundance. In the example of boron-10, you would divide 23 by 123 and multiply by 100 to calculate a percent abundance of 18.7 percent. Likewise, boron-11 would have a percent abundance of 81.3 percent.