Look for the Qi standard
Apple has made iPhones compatible with the Qi charging standard. This is not surprising: Qi is by far the most popular standard for wireless smartphone chargers, and you can find countless models that use Qi.
However, it’s important you double check to make sure, on the chance you’re looking at one of the few chargers that uses a more proprietary system.
Look for phrases like “Qi-certified” or “Qi-enabled” that let you know it’ll be compatible with your iPhone.