Ideally you notice it from the shore and avoid it as others have mentioned. If you’re swimming in one you’ll realize soon enough you’re getting taken out from the shore.
I got caught in one when I was a teenager off Mission Beach in San Diego. I’d already been out swimming in water deeper than I could stand in for a while and, getting tired, started heading to shore. After some time I realized I wasn’t making any progress at all. It took me a little while more to realize what was up: I was in a rip current. Thankfully I’d had an elementary school teacher in Phoenix, AZ of all places that taught us kids how to escape one and I remembered - swim parallel to the shore a good distance and then try swimming back in again and check your progress. Repeat as needed if you’re unable to make progress. I followed those instructions and eventually was able to get back to shore, utterly exhausted. I can’t help but think how lucky I was not to have missed that day of school.