Checking the Logs...HOW TO??

Note that if you're having problems with pool members unexpectedly changing state, then consider also sol12531 "Troubleshooting health monitors", which gives you some detail on log messages and how to get more debug logged.

Pertaining to tcpdump, also note sol13637 "Capturing internal TMM information with tcpdump" and the very nice "p" option:

"Beginning in BIG-IP 11.2.0, you can use the p interface modifier with the n modifier to capture traffic with TMM information for a specific flow, and its related peer flow. The p modifier allows you to capture a specific traffic flow through the BIG-IP system from end to end, even when the configuration uses a Secure Network Address Translation (SNAT) or OneConnect."