File existence can be validated only server-side. You can use the FileExists server event of the RadUpload control to check whether the corresponding file exists and display a message which prompts the user to choose whether to override the file or do not save it. I suggest you take a look at the FileExists help aricle
for reference. However, in both cases the file will be upload to the server and the depending on the user's response the existing file will be overridden or the file will not saved.
All the best,
the Telerik team