On iPhone X and later, you can see the battery percentage in Control Center. Just swipe down from the top-right corner of your display.
On iPhone SE (2nd generation), iPhone 8 or earlier, iPad, and iPod touch (7th generation), you can see the battery percentage in the status bar. Go to Settings > Battery and turn on Battery Percentage. When you use Low Power Mode on your iPhone SE (2nd generation), or iPhone 8 or earlier, the battery percentage will always show in the status bar.