TL;DR Reverse Image Search
So you're looking for a place to live online and you come across this sweet apartment with a perfect location that's well within your budget. You're either doing well financially, you want to live in a shit hole or someone is trying to scam your ass. How can you know for sure?
Generally scammers tend to just grab their images from real ads they've found elsewhere online, so one really good way to check is doing a reverse image search either on Google Image Search or Tineye to see if this is the case.
Just "right-click" one of the images from the ad and copy the URL / address of the image, then paste it into either Tineye or Google Image Search. On Google Image Search you will need to click the camera icon in the search bar and select "Search by image".
You will then get a bunch of results of images that look visually similar to the one from your ad. Just browse through the results and see if you can find your exact image posted elsewhere.
If you can find the image being used for a completely different listing somewhere else - then the ad you had been looking at was likely a scam. If you can't find it - it doesn't necessarily mean it wasn't a scam, but at least you've checked this.
If it looks too good to be true it probably is.