What is the best lab test to check adrenals?

No single test. The adrenals make quite a few different hormones, so no single test can tell if everything is just right. And it gets even more complicated. One key adrenal hormone, cortisol, can vary a lot between normal people, and within the same person depending upon time of day, and whether or not they're feeling well or not, so that different tests are needed for suspected high or low cortisol levels.