Banana bread is one of the easiest and tastiest things you can bake, but it's not foolproof. If you suspect you've underbaked a loaf, perform this simple test to find out.
When you take banana bread out of the oven, insert a toothpick into the center. If the toothpick comes out wet, then the loaf is underdone and will be gooey on the inside.
If the toothpick comes out mostly clean but with a few crumbs stuck to it, you'll know that the banana bread is perfectly done and neither under- or overbaked.
You can also use a food thermometer to measure doneness. Properly baked banana bread will have an internal temperature of 200 degrees to 205 degrees Fahrenheit.
Remember that you can bake the bread for longer, but there's no undoing a dry, overbaked loaf. Try periodically checking the bread with a toothpick or thermometer until it's done.