If you know the variety, and esp if it hss flowered before, trunk height is a very accurate predictor. The Cavendish varieties end to have a group of leaves without adding much trunk height (the oleaves are somewhat all bunched together), when they are close to flowering. My Dwf namwah behave similarly. Heivght is the most discernable feature, if you know what the height should be (based on experience at your location, with your climate, soil and nutrition conditions). Also, sometimes they get "floppier" or tend to start leaning over when the flower begins to push up through the trunk, esp on the skinnier varieties.