Written by Philippa Law and Caroline Bannock

Storm force winds and rain brought flooding and power outages to the south of the U.K. in October 2013. This event affected a lot of people, so to widen and enrich the Guardian’s coverage, we asked our readers to share their photos, videos and stories of the disruption via our user-generated content platform, GuardianWitness.

Among the contributions we received was a bizarre photo of what appeared to be a giant multicolored beach ball, at least twice the height of a double decker bus, on the loose at Old Street roundabout in London. This was one of those images that immediately evokes the question, “Is this too good to be true?” We were very aware that it could be a hoax.

We started verifying the user’s photo by running it through Google reverse image search and TinEye to verify that the image hadn’t been borrowed from another website. Users often try to show us a news event by sending pictures that have been published on other news sites, or shared on Twitter and Facebook. So a reverse image search is always the first check we make.

In the case of the rampant inflatable, Google returned no hits - which suggested the photo was either original or very recent and hadn’t been picked up by any other news organizations - yet. Good content gets published very fast!

The most important verification tool we have is a direct conversation with the user. Every contributor to GuardianWitness has to share an email address, though there’s no guarantee it’s a correct one. So we emailed the user in question to try to make contact. In the meantime we continued with our verification checks.

Usually we would verify where a photo had been taken by comparing it with images on Google Street View, but as our team is familiar with the Old Street area, we recognized the view in the photo and felt reasonably confident the picture had been taken there. Although we knew the area, we didn’t recall seeing a giant beach ball - so we searched online for earlier evidence. We found it had previously been tethered to the top of a building nearby. This finding meant the image was looking less like a hoax than it had first appeared.

We checked Twitter for mentions of the beach ball that morning and were able to confirm that there had been other sightings around the time the user claimed to have taken the photo. Our Twitter search also revealed a later photo, taken by another user, after the ball had deflated.

Finally, the user got in contact with us and, by speaking to him on the phone, we were able to confirm that he had taken the photo himself.

Having taken all these steps to verify the image, we were happy that the story held up to scrutiny. The compelling image of a runaway beach ball in the driving rain was published on the Guardian’s live-blog and was shared widely on social media.