There are a few different ways to identify which Delta aerator you need. The first way is to check the model number of your faucet. This can usually be found on the underside of the faucet handle or on the back of the spout. Once you have the model number, you can then consult Delta's online aerator guide to find the correct aerator for your model.
Another way to identify which Delta aerator you need is to take a look at the existing aerator. If it is a Delta aerator, it will have the Delta logo stamped on the side. You can then consult the Delta online aerator guide to find the matching aerator for your model.
If you are still unsure of which Delta aerator you need, you can always contact Delta customer service for assistance.