One needs a clinical suspicion of Cushing's syndrome and the 24h urine test can support this with evidence of high cortisol levels.
Then it needs evidence of elevated, somewhat autonomous, cortisol levels and a reliable way to do that is with a dexamethasone suppression test, which takes 2 days.
If that test shows cortisol that fails to suppress after the dexamathasone, then one must measure cortisol and ACTH (at 9am). The ACTH is quite difficult to sample and measure, it usually takes a week or two from when it is taken. If the cortisol is high with detectable/ high ACTH then one must go on to look where the ACTH is from. This usually requires sampling from the blood veins that drain the pituitary; it is a very specialised test and can take weeks to set up in one of the centres that do it, often needing a repeat evaluation before performing it +/- CT chest and MRI pituitary.
Cushing's disease can be very difficult to diagnose.