If you need to check a particular aspx file is exists or not in your project/web site then first you have to get the root directory physical file. After that you can check it with Directory object that the file is exists or not. The code example like
protected void Page_Load(object sender, EventArgs e)
{
bool found = IsFileExists("abc.aspx");
}
private bool IsFileExists(string fileName)
{
string rootWebDirectory = Server.MapPath("rootwebdirectory");
string[] files = System.IO.Directory.GetFiles(rootWebDirectory, fileName, SearchOption.AllDirectories);
return files != null && files.Count() > 0;
}