I can't say for certain as I don't currently have access to a v8 Firewall running sendmail with TrustedSource enabled - or GTI as it is now called. But, I would expect to find it in one of two places:-
Either in the main sendmail logfile /var/log/maillog, or in the Firewall's primary audit feed - the /var/log/audit.raw file or by running the audit viewer. In the case of the audit, I would look out for events associated with the sendmail process (they may well be 'attack' events) when connections to sendmail are being blocked by TrustedSource/GTI. When you open up any of these audit entries and look at the data inside, you should see the reputation score contained within it.
Hope that helps.
You could try these acat commands on the command-line:
$> acat -e "category AUDIT_C_GTI"
$> acat -e "event AUDIT_R_REPUTATION"
or (both together)
$> acat -e "event AUDIT_R_REPUTATION or category AUDIT_C_GTI"
I am not positive those will work as I do not have any examples of emails denied by TrustedSource/GTI to look at myself.
The more I think about it the more I recall looking at a customer's appliance, I could see evidence of messages being blocked in the maillog file.