How do i know if the test is correct?

I have this doubt in mind. If I want to create a program to make sure that my previous program does the increasing sorting of an array correctly. So doing it, I would not be directly creating the program I want to do ?.So my doubt is... How can I make sure that my program tests if the sort executed is correctly? and if I know the test is correct,I would not have directly created the program I wanted to create at the beginning?

I know it's strange and I hope you have understood the concept. It's like the concept who debug the debugger?