There are two ways: literally walking up and asking or using the digital waitlist in your
MDE. To access that, you'll click the + sign to book dining but choose "now" as the timetable. It'll show you any restaurants with open waitlists. Note, you have to be close enough to one for it to allow you to join it. You can open up your app and see now what restaurants have open waitlists and which are at capacity.
Some restaurants, I've found, never have capacity. Topolino's is one I've never seen available. You are much much more likely to dine there if you start looking the day before for a reservation. We've eaten breakfast there 5-6 times and 100% of them were booked the night before.
That said, each time I've been they have been accepting walk-ups in person to sit at the bar. Characters didn't seem to go over there (but maybe they do?) and to be honest, I only saw some people clearly staying at the hotel on business wanting breakfast.
If you're staying there, never hurts to take the elevator up and ask. IME, it's unlikely unless you want to sit at the bar. We have had a decent wait (once 45 minutes!) nearly everytime we eat there which makes me think they are booked solid all the time.
