Nowadays, it becomes more and more difficult to tell whether a photo has been retouched, tampered with or not, this being due to the evolution of the technology of image editing, knowledge, internet, etc. Photo editing is becoming almost too simple these days, with a smartphone and a good app, it's very easy! It is no longer really necessary to use the famous Photoshop software as it used to! Admittedly the quality is not the same (and again, it depends on the time we spend, the knowledge and skills of the retoucher) but I would say still that it is much better than if we passed via Paint which offers almost nothing! :-D
How do I know if an image has been retouched?
A site can tell you if an image has been retouched or not: Forensically Forensically offers several tools that will let you know if a photo has been retouched or not, after sending the photo to their site. Each tool has its adjustable settings. Among the tools, you have among others the magnifying glass, noise analysis, clone detection, etc… Some examples of retouched photos and what forensically can give: the site is very easy to use, the whole being to quibble in the different settings to know if a photo has been retouched or not. Forensically site: https://29a.ch/photo-forensics/#forensic-Magnifier
Detect a retouched photo with Google!
Use Googles Images! I told you how to search for an image with an image on Google, eh well for this, Google Images, by its excellent algorithms allows you to find images similar to the one you submitted! Therefore, if you find a similar image, but different, it is that one or the other has been retouched!