I have similar health concerns and after getting some bloodwork back showing low adrenal output, I too had some questions about the possibility of Cushing's.
Here are some specific things you can do:
1. general bloodwork will show the overall adrenal output, if it is high or low
2. a specific lab test called an "adrenal stress index" (ASI) can be performed to determine which part of the adrenal glands is putting out too much/too little hormone--the medulla and the cortex produce different hormones, so it's important to know where exactly the balance is off. The ASI is not a standard test, so you may have to have it sent off to a special lab. The out-of-pocket lab fee for the ASI is about $100.
3. If you would like to take some general measures to support adrenal health, without having to shell out money for special tests, there are two important herbs for the adrenals: siberian ginseng (powder form or tincture) and deglycerinized licorice, either capsule form or solid extract (a gooey mess you can mix into hot water or eat from the spoon which, if you like black licorice, tastes fine).
Also, caffeine and sugar are big no-nos for people with adrenal exhaustion or other adrenal concerns. Low carbing is good for folks with pcos, by stimulating ovulation through regulation of insulin release, but it is also good for the adrenals, which would really enjoy relaxing once in a while, rather than constantly being pushed to their limits.
Stress reduction, obviously, is another big component. If you have a tendency to worry a lot or let things upset you easily, chances are, your adrenals are putting in a lot of overtime.
No, I am not a doctor, just a nerdy lady with health concerns...but be that as it may, I hope that these ideas may be of help to you.
Best wishes to you as you search for answers.