I went to a filming of the Mickey Mouse Club (the one in the 90s) when I was a kid. You have to get tickets and it is planned in advance. There is not an option to sign up when you get there. MGM, or whatever it is called these days, used to have an audience for filming fake shows. They have actors that look similar to the celebrities and acted like they were filming a show. There are age restrictions for who can attend a real filming. I don t know if you have ever been to a filming before, but we were there for 3 or 4 hours. We sat quietly until they told us to clap. They filmed each scene 15 or so times.
There no real to find out, its just a matter of luck if your at the right place at the right time. Disney does not publicly disclose that type of information as as they need to protect the privacy and security of the celebrities.
It is hit and miss.I have seen Leonard Nimoy and Sean Astin there. Both at Hollywood studios, and on the same day.I also saw Michael Jackson and Rod Stewart at Disneyland once. Those were on different visits.
There is no way you will know if a celebrity is going to be in Disney unless they are filming the Christmas Parade, which has already been done for this year.
Only I think of is checking disney world websites or check your favorite disney stars websites. Watch an entertainment news show