Is there a simple way to check if a file exists in a folder on the user's hard drive (standalone game) without having to create an array of all files in the folder and iterate through it?
You can use File.Exists:
http://msdn.microsoft.com/en-us/library/system.io.file.exists.aspx
Funny enough, the usage is identical in C# and JS
if (System.IO.File.Exists("myfile.txt"))
{
//do stuff
}