Monitoring Syn cookie threshold in real time

Viewing virtual server statistics from TMSH should give you the information you are looking for: tmsh show ltm virtual <virtual-server-name> detail shows the following SYN Cookie statistics. (Note: This output is from a virtual server that does not have SYN Cookies activated at the moment, hence the 0 values.)

SYN Cookies
  Status                         not-activated
  Hardware SYN Cookie Instances              0
  Software SYN Cookie Instances              0
  Current SYN Cache                          0
  SYN Cache Overflow                         0
  Total Software                             0
  Total Software Accepted                    0
  Total Software Rejected                    0
  Total Hardware                             0
  Total Hardware Accepted                    0