No way to discover if a byteman agent has already been installed.

The agent sets a static boolean once it loads the first time to true (see org.jboss.byteman.agent.Main class). However it's not discoverable from somebody outside the JVM if it has been attached already. For example if somebody used the Install API to inject the agent into a running JVM there is no way to detect this once done. What's more an exception is thrown if one attempts to attach an agent more than once.