Availability Group - how much data was lost after forced failover
Go to SSMS
Object Explorer
Expand Always On High Availability
Expand Availability Groups
Right click your AG
Select Show Dashboard

In the dashboard, above the list of AG databases and their status, there is Add/Remove Columns button
Pick Estimated Data Loss (time) and add this column to dashboard
You will be able to see the estimated data loss time for each DB
You can check this dashboard right before you are going to do forced failover, to get the idea of how much data can be lost