how to check existence of file and folder in a special direction(not the current folder)!?!

I am making a GUI! Then I want to generate an .axe file of it. This program should save some data by format .txt in a special direction that the user will specify. The program should check the file names in that folder not to create a file with same name! I think the command exist() can't be useful here because it is useful for current folder!