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A competition I want to participate in demands that they want unedited photos. The submission should not even have basic edits like brightness, saturation, white balance etc
They claim they'll check this using meta data, is it possible that they'll know if one has edited the photo?
I think some people might edit and submit and will get an unfair advantage...
Being slightly harsh, competition rules like that show that the organisers don't really understand how modern cameras work. A very high level and simplified view of how a camera makes a image (JPEG):
Submissions for these types of contests, and even some news agencies, require photos to be submitted as straight-from-camera JPEG files, not as exports of RAW images. This is usually enough to satisfy the submission requirements.
Most of the time, people trying to skirt such rules by faking EXIF data, tend to make a mistake somewhere, that is a tell-tale clue of manipulation. Often those clues are discrepancies in the various time/date stamps in the EXIF data and/or the filesystem metadata. Sometimes there are extra EXIF fields modified or added by their manipulation tool, that they weren't aware of.
To the point that even straight-out-of-camera JPEGs are manipulated by "decisions" in the camera's RAW processing algorithms, that ignores intent: a CPU or camera has no intent or goal; it is merely a complex machine following its programming. Only a person has intent, so implicit in the requirements for your contest are "no intentional edits" to be made.
But in a strict sense, there is no technical way to be absolutely sure an image wasn't manipulated. Because at some point along the way, we're out of the realm of technology, and into the realm of trust.
For smaller competitions, the stakes aren't worth it to ensure a high degree of trust. That is, the contest generally trusts (but attempts to verify) submissions were made according to the rules. But they're not going to employ expensive time-consuming techniques to try to disprove the trust. They will use simple, point-and-click -style tools to pass a "good enough" test. That's all they can do. As the stakes of the contest increase (such as substantial monetary rewards, etc.), the techniques and tools to spot alterations can be more sophisticated and more expensive.
For some news agencies, they actually extend the trust even quicker that submitters don't alter their images. This is because the photojournalists have a reputation to stake (as do the news agencies themselves), coupled with contractual submission agreements. Those agreements are backed with the possibility of termination or severance of relationship if undisclosed manipulations were made. See What are the editing restrictions for sports/photo journalism?
For other purposes where image integrity is absolutely important, knowing that technological means cannot guarantee image integrity, the trust is placed in processes and procedures ensuring the chain of custody, file handling, etc., is maintained, so that the opportunity to alter the image is reduced or eliminated. Things such as file checksumming, append-only data stores (i.e., blockchains), can certainly help, but they are not guaranteed to ensure unmanipulated data if there is no knowledge of the chain of custody of the data before those tools were employed.
An additional thing to take into account is the code that creates the jpeg has an identifiable signature. The way Photoshop writes the jpeg will be different than the way Corel will write it. Exiftool, for example, has a JPEGDigest tag, which is defined as:
an MD5 digest of the JPEG quantization tables is combined with the component sub-sampling values to generate the value of this tag. The result is compared to known values in an attempt to deduce the originating software based only on the JPEG image data.
JpegSnoop goes into more details on it's Identifying Edited Photos.
By looking at the exif data, it's possible to know if the image has been developed in lightroom, or comes straight form the camera as a jpeg. This can also be faked.
Now, if you take a picture, then look at the result on the camera LCD, you can check the exposure, histogram, white balance, etc. They you can adjust settings and take another picture. Technically, it is not edited, but the result would be the same as if it was. You could also use bracketing.
I think this rule somewhat gives an advantage to photos of subjects that won't go away, like landscape or portraits, since these subjects allow several attempts. If the intent of the rule was to make the photos more spontaneous and candid, it may achieve the opposite.
