In an attempt to tell whether the tumor is in the pituitary or in the adrenal gland, the vet may attempt a high dose dexamethasone suppression test (about 80 % of Cushing's dogs have the tumor in their pituitary gland). (2ndchance.info)
CBC / WBC and blood chemistry values including a blood glucose screen for diabetes (dog)(cat), cholesterol level, Thyroid panel for hypothyroidism (free T - 4), ACTH stimulation test or low dose dexamethasone suppression tests for Cushing's disease, TLI, cPL ® or fPL ® for Pancreatitis, fructosamine test for diabetes, urinalysis including protein: creatinine ratio, microalbuminuria test (2ndchance.info)
The other is an overnight dexamethasone suppression test (DST) that causes the production of cortisol to decrease in healthy horses, but not those with PPID. (sciencedaily.com)