Once you get your program to compile with a C++ compiler, put it on http://ideone.com (which happens to be an online compiler that allows arbitrary input) and give it the input similar to what your contest gives you.
For example, run it with
in the input field, and if you wrote it correctly, it should show
This is where you have to be aware of the limits of your data types - you need a different method because with the standard types you are not going to get anywhere near factorial 100. Even a 64 bit unsigned int only gives 20 significant figures.
..oh, right, LWS allows input too now.
Calculating factorials up to 100 is not a one-liner program in C++, that's what makes it a competition, I guess (in practice, it's trivial with boost-1.53 or with other multiprecision libraries)
Or you could just hardcode all 100 possible answers as an array of strings.