F5 logs are available under /var/log/ directory.
If you are looking for LTM traffic related logs, look under /var/log/ltm
If you are looking for GTM traffic related logs, look under /var/log/gtm
GTM has been renamed as DNS from 12.x code version.
The logs are rotated every 24 hours. There will be one ltm file that contains the latest logs and multiple ltm.gz files like: ltm.1.gz, ltm.2.gz etc. The ltm.1.gz files can be opened by using the following command: zcat ltm.1.gz
(tmos)# run util bash ~# cd /var/log ~# cat ltm
Other such logs are outlined in this article – K16197