The bed's not made in the photo
This is the number one red flag. If your host is too lazy to properly lay out a bedspread for the sole purpose of advertising their space, tidiness obviously isn't a high priority, and neither is your vacation/bachelor party/bed bugless future.
There are five photos of the same room
It's great to know that there's no mold in the shower, but if a photo gallery leans too heavily on one room, odds are the others aren't nearly as fungus-free.
The photos appear to have been taken on a '90s flip phone
Not everyone knows to take Airbnb up on their offer of free professional photos, but if the images are as grainy as rough sand, the sheets might be too.