The way you can tell if it's an Autumn Fern is because they typically have an odd leaf that is sort of yellow/orange. They're not ALL green. Which is why we use them a lot in planting, because they offer some color other than green
and are mostly evergreen.
Often, they'll look a little more like this:
Warning though: If it is an autumn fern you are seeing, they aren't quite as hardy as many other ferns. They can die totally in the winter if it gets too cold or totally die back to the ground and then come back in the spring.