how to detect potential conflict with other iRule?

There is no built in way to determine this but you can add the logic to your iRules with session variables.

The following iRule has two HTTP_REQUEST events with different priorities. The lower priority (100) will execute first. In the first iRule, I set a session variable ALREADY_RESPONDED to 1.

iRule 1:

when HTTP_REQUEST priority 100 {
  set ALREADY_RESPONDED 1
  HTTP::redirect "http://www.foo.com"
}

In the second iRule, check to see if that ALREADY_RESPONDED variable has be set to anything using the "info exists" command. This is basically equivalent to a "is not null" comparison.

iRule 2:

when HTTP_REQUEST priority 200 {
  if { ![info exists ALREADY_RESPONDED] } {
     HTTP response not yet sent, continuing with iRule...
  }
}

Keep in mind that for keep-alive connections, you may have to "unset" the variable in the last iRule in the chain so that future HTTP requests over the same connection don't inherit the session variable from the previous request.

-Joe