They can tell your non-muslim by identification, i.e identity card, visa's etc. The saudi governemnt just won't allow non-muslims in at hajj time, if there are non-muslim (illegal immigrants, crossing borders etc) , they will be isolated until the hajj period is over. the only non-muslims who can get in are tv stations, media, news crew etc, but they get permission from the saudi goverment.
hope it helped answer your question
zahid