have to check some formulae whether they are providing correct result, these formula are written part of a VB program, what is the best tool\method to unit test these formula?
There are many unit testing frameworks out there, ultimately you will need to choose the one that works for you:
http://en.wikipedia.org/wiki/List_of_unit_testing_frameworks#.NET_programming_languages
Typemock is very powerful, NUnit is probably the de-facto standard.