how to identify when the last update or insert was done on a particular table... is there a way??
In general - no.
Usually it is good habit to maintain this info in some meta data columns / tables during the ETL process.
LastAccessTimestamp from dbc.tables will not help you as it covers also selects.
If all queryies of users which are able to do updates and inserts on the tables in question are logged via DBQL you can search there for Update or Inserts.
What about the LastAlterTimeStamp?? Will it consider only insert and update or select also??
from the manual
LastAlterTimeStamp Returns the time the Data Dictionary row was last updated.
This will be done if the table itself will be changed for example with an alter table statement.
So no - will not help