Easiest way:
1. You would have tuned your JVM heap for the rest of the nodes. Sat 1 GB or 1.5GB etc
2. Generally people do not tune the JVM heap for admin server since it doesn't have any heavy duty operation. So it would be defaulted to the standard memory of 512 or so.
Now, you can search for the java process with 512 MB (provided these are the only java processes you have).
Second way:
1. Check for the admin port and find out your process.