If you can describe a language with a context free grammar, then the language is context free. It would be difficult to prove a language is context free using the pumping lemma, because even if you find a string that can be pumped, there still might be a string that cannot be pumped.
You usually use the pumping lemma to prove a language is not context free. Because all you need is one example of a string that cannot be pumped.
Here is an example of a string in L = {0^i 1^i | i >= 0} and how it can be pumped.
string w=01, can be split as follows:
u = empty
v = 0
x = empty
y = 1
z = empty
u v^i x y^i z is in L for every i >= 0