Smell the crust first. Roasted wheat? Caramel? Vinegar? All information.
Bite through the crust. Does it shatter, tear, or crack? Each is a different bake.
Chew slowly. Sourdough should have depth — tang, nuttiness, sweetness in that order. A one-note bread is usually under-fermented.
Notice the finish. Great bread lingers. Mediocre bread disappears from your mouth the moment you swallow.
Do this with three loaves from three bakeries side by side. You'll never eat bread the same way again.