The recommendation from the Endocrine Society in the USA in collaboration with the European Society of Endocrinology says the following:

R 2.4 We recommend against routine testing for adrenal insufficiency in patients on supraphysiologic doses of glucocorticoids, or if they are still in need of glucocorticoid treatment for the underlying disease.
https://www.endocrine.org/clinical-practice-guidelines/glucocorticoid-induced-adrenal-insufficiency
-----------------------------------
I had a routine a.m. cortisol level done when I was on 3 mg of Prednisone. I didn't need Prednisone anymore for PMR because Actemra was controlling my PMR symptoms. I had symptoms consistent with adrenal insufficiency. I did not have a Synacthen Test which I think is the "dynamic test" they are referring to in the following recommendations.

R 2.7 If confirmation of recovery of the HPA axis is desired, we recommend morning serum cortisol as the first test. The value of morning serum cortisol should be considered as a continuum, with higher values more indicative of HPA axis recovery. (⊕○○○)

R 2.8 We suggest against routinely performing a dynamic test for diagnosing adrenal insufficiency in patients tapering or stopping glucocorticoid therapy. (⊕○○○)
------------------------------
My first morning cortisol level was "low" and the following guideline was followed:

"We suggest that if the result is < 150 nmol/L or 5 μg/dL, the physiologic glucocorticoid dose should be continued, and the morning cortisol repeated after a few months."
--------------------------
I was told to stay on 3 mg of prednisone and referred to an endocrinologist. It was the second time I was referred to an endocrinologist. The first time was when I was still taking 10 mg of Prednisone. The endocrinologist said a cortisol level wasn't warranted because 10 mg of Prednisone was too much for a reliable cortisol test. I was told that I needed to be on 3 mg of Prednisone or less. The endocrinologist referred me back to my rheumatologist the first time to see what could be done to lower my Prednisone dose.

My second evaluation by an endocrinologist was an hour long. My symptoms were reviewed. My endocrinologist wasn't surprised by my low cortisol level because I took Prednisone for 12 years to treat PMR.

According to the "essential points" in the above link:

"Suppression of the hypothalamic-pituitary-adrenal (HPA) axis is an inevitable effect of chronic exogenous glucocorticoid therapy, and recovery of adrenal function varies greatly amongst individuals."