I had water leaking into my chain case, to figure out where it was coming from and confirm the repair I used a Smoke Generating machine used in my shop for locating Evap or exhaust system leaks. I plumbed it into the fill plug at the front of the case. Even without a smoke machine, I suppose you could plumb just low pressure air to yours and listen for where it is escaping. This might be better done with an assistant to control/meter the airflow to prevent pressurizing the case and blowing a seal out.
It worked for good for me, my machine had multiple leaks from different access covers, once I had them all fixed I knew it was a done deal.