I am not sure if there is a way currently.. There might be some way in the old 1.4 API? You could try looking....
Otherwise, this might be a bit of a manual slog to figure out who has access to what.
You might be able to also look at the access logs (while this won't show who has access, it might at least show who is actively viewing those dashboards?
The logs can be found by going to:
Admin > All Admin > Logs
There are three tabs here: Admin Tools, Usage & Access Log and Report Suite Change Log
I believe there is a filter for Event Type = Dashboard.....
Good Luck!