Is there any way in an iRule to tell if APM is enabled or disabled on a connection?

If you are using ACCESS::disable then the easiest way would be to set a user defined tcl variable to indicate the state. I have found no combination of built-in commands that is completely error proof or that require complicated parameter passing.

ACCESS::disable
set access_disabled 1

if {$access_disabled} {
     Do Stuff outside of APM
}

You might be able to get ACCESS::session sid working but this also requires that you code an if check to do a sting comparison which I guess would be more CPU expensive than doing an integer check with your own variable.