How do you find the atomic mass of an element?
You can easily find this property from the Perodic Table of the Elements, from Wikipedia, and many other sources, given the element name or symbol.
The atomic mass of an atom can be measured (weighed) with an amazing precision by means of an apparate that is called "mass spectrometer".
Thus, you recognize the value of atomic mass because it is has several digits; you can't mistake it with atomic number, that is an integer.
If the element is a mixture of isotopes, as occurs in most of cases, the atomic mass actually refers to the average of the atomic masses of the several isotopes of that element, given that the percentage of the different isotopes of an element is roughly constant around the Earth crust.
For example, if you take one hundred thousand carbon atoms at random from any part of the planet, you will get between 98853 and 99037 atoms of the most common isotope C-12 (having atomic mass exactly 12.00000 atomic mass units), and between 1147 and 963 atoms of stable isotope C-13 (which have one more neutron in their nuclei), whose masses are 13.00335 atomic mass units. (source: http://en.wikipedia.org/wiki/Isotopes_of_carbon)
Let's take the weighted average in both sides of the interval:
(12•98853 + 13.00335•1147)/100000 = 12.01151 a.m.u.
(12•99037 + 13.00335•963)/100000 = 12.00966 a.m.u.
So, the average atomic mass of carbon oscillates between 12.01151 and 12.00966, depending on the place were this element is mined from.
The value that is reported on the PTE is generally 12.011 or 12.01. It represents the central value in this interval.
The atomic mass of those elements that are formed by a single isotope are given with more digits because there isn't any possibility of isotope composition variation for them.
For example, the atomic mass of sodium, Na is 22.9897692809 a.m.u.