how to prove fallback host triggered is due to 1 bad request respond from backend server?

another thing I want to confirm is:

irule change only applied to new connection.but how to check the existing connection which use previous irule .

It affect the site for many pages since akamai keeps long connection with us,they don't create new connections for many hours ,I don't want to restart all these current connections ,just want to evaluate the impact.

I open a case with F5 TAC,but it seems it is difficult to find the connection which use previous irule .....

not sure whether I clarify my question here

correct me if I am wrong