There basically are two ways:
Number one: Set up .htaccess password protection. That way, if you try accessing the directory, in which the wiki is stored, you will have to enter username and password. If you do not have valid credentials, you will only see an Apache error page - no output will be produced by the wiki. So from the server output, there is no way to tell, what is behind that protection. You will easily find tons of tutorials on the net on how to set up .htaccess and .htpasswd to protect a directory.
Number two: Configure the wiki to only allow anonymous users to see the wiki login page, but no other wiki pages. It will be clear that a MediaWiki installation is running there, but the content will be hidden just as in number one. A guide for this can be found here: Manual:Preventing_access#Restrict_viewing_of_all_pages.