is there any quick way to get a list of all beastmodes that have been shared on a dataset?
Not that I'm aware of, but beastmode administration is something they're working on, and the "shared" flag is probably an attribute available in it.
Your current workaround would be to just go to a dataset and open a card and evaluate each beast mode individually to find whether the checkbox has been set.
I just recently discovered a "Domo Governance Dataset" within the list of connectors. It looks like there is a Beast Modes data set that could contain this information.
However, it looks like you can only access the data if you have administrator acces.