The most common technique in ensuring that something like a digital photographic image is authentic is to have it encrypted at the time it's taken, and this is the technique that's used in criminal investivations where images are presented as evidence in courts. For what I know none of the major DSLR camera manufacturers like Canon and Nikon offer such features built-in to their cameras. However, Canon offers an accessory called Original Data Security Utility or OSK-E3. However, this system has been exploited four years ago.
The programmer and encryption expert is Dmitry Sklyarov, and his
company, Elcomsoft, has found a vulnerability in Canon's OSK-E3 system
for ensuring that photos such as those used in police
evidence-gathering haven't been tampered with.
The result is that the company can create doctored photos that the
technology thinks are authentic. To illustrate its point, it released
a few doctored photos that it says passes the Canon integrity checks.
I'm not sure if it's based on any additional hardware component or if it's only software based, but Nikon also has an authentication system (Image Authentication Software), and their system has also been exploited by the same people the following year.
So in conclusion, there's no way to be absolutely sure that the digital photos that are presented to you have been tampered with in any way. The best thing to do is to look for visual evidence in the photos. You would need to do some serious, in-depth data analysis to be sure, and even then, your findings might be inconclusive.
In this situation, I would suggest you use common sense when you try to decide whether or not a photo has been tampered with or not. What does the photo look like? What do you know about this person? Is he or she really that good photographer?
To me, it sounds like you are already doing a good job. Because you have already found some tampering attempts. If the metadata - the details about an image, like the date and time it was captured - is the major concern, you should know that this is the easiest thing to manipulate in a digital photo file.
One thing you can do as part of the validation process, is a reverse image search, a type of content-based image retrieval (CBIR). I recommend using primarily Google Images, but also TinEye. You can pick one of them, or use both. In my own experience, TinEye is sometimes able to find images that Google cannot, and it can take Google up to two days before they have indexed these and they too can find the same images as TinEye.