It's easier to identify a jellyfish sting when you can see the tentacle trails in the reaction. Some jellyfish-like creatures (man-o-war or blue bottles, for example) have thicker tendrils that cause a much wider reaction.
There is a lot of debate about the best way to treat jellyfish stings, and non-jelly critters like the man-o-war make the debate even more complicated. For now, the two most accepted treatments are rinsing with hot water or with white distilled vinegar.
For more jellyfish treatment steps: How to Treat Jellyfish Stings.
Have a jellyfish sting? Take a picture and submit it here.
Any opinions expressed here are for educational purposes only and are not intended for diagnosis.