Sets can usually be timed, you just have to watch them for a while. Sometimes sets from two different swell directions can make it pretty tough though. It's tedious to watch for twenty minutes too...but often you can find a consistent rhythm. I usually watch for a minute or two and take my chances uninformed.
Hope someone finds (and returns) your gear...hasn't happened yet, but I usually have a mask & snorkel in case of personal yard sale.