Just go to your local martial arts school and ask them if they have any black belt tests coming up. In most disciplines the black belt tests are comprehensive and gives you a great overview of the style. Typically they cover all the forms they learned up to that point, some sparring, and if the style does board breaking they'll do some of that.

Some styles don't quite follow those patterns, but can be entertaining to watch anyways. The Akido black belt tests are like watching a violent dance.

My personal preference is to go watch a black belt/sash test at a local Kung Fu school (not Tai Chi or Wing Chun). Usually you'll see about a dozen really interesting forms, including a variety of traditional weapons, and two and three person forms, and some sparring. Typically you won't see board breaking in Chinese styles. If the style involves Chin-na you'll see some joint locking and maybe wrestling. It's kind of a tour of 1500 years of martial arts history.

At one school I went to, the test was about 4-5 hours long, and continuous (no breaks) -- apparently the student had also undergone a 1 hour fitness pretest before qualifying for the test which included some crazy metrics like 100 v-sits, 100 push-ups, 15 minutes in a low horse stance and some other things.

These guys were very fit.

They would be pretty rare on youtube I expect, the sensei's i have encountered have always been pretty secretive. I would suggest just starting up (they love helping beginners) and going to the first grading to see what it's like.