Easiest way to check if an arbitrary String is a valid filename

Check against GetInvalidFileNameChars():

var isValid = !string.IsNullOrEmpty(fileName) &&
              fileName.IndexOfAny(Path.GetInvalidFileNameChars()) < 0 &&
              !File.Exists(Path.Combine(sourceFolder, fileName));

Check whether filename.IndexOfAny(Path.GetInvalidFileNameChars()) < 0 and !File.Exists(Path.Combine(someFolder, filename))