I used to work extensively on burial sites in the US, so while I can't speak specifically to how Canada will be approaching these excavations/examinations, I can speak to the process more generally.

1. archival/cultural research. One of the first steps is to research the area, including archival (document research) and cultural (more or less, speaking to locals, including owner/managers of the property. Often this will tell an archeologists where a burial site is. In this particular case, my understanding is that these are cases where the burials would not be necessarily recorded though;

2. surface testing/GPR. GPR is not my specialty, but I am technically trained in its use. People I would consider an expert in GPR can often identify a potential burial up to 10 ft. However, it's often not clear on the imaging screen that you are looking at a burial. It could be some sort of other object resembling a burial. Burials can also be discovered by digging test pits in an area where burials are believed to be. Finally, many burials are discovered during standard construction practices. Not ideal, but extremely common, especially in the US.

In this particular case, it sounds like there is a general idea where these mass Graves are, so something like an excavation test pit (excavated by hand tools, usually) will probably be the primary means of burial identification.

3. when it comes to dating remains (or any archeological evidence), there are few methods, which I'll attempt to explain clearly (apologies if it's not). In this particular case, they can a) test the remains themselves, probably by using a tooth. This is the most accurate way to date something. However, it's requires the sample to be destroyed. When working with human remains, destruction of said remains is not always possible, especially when working a site with political ramifications, or in situations where religious matters are involved (as is often the case with indigenous/native burial sites. Many consider this form of aging to be very offensive This is an absolute dating method, meaning that the age of the object can be absolutely identified (within a time frame, depending on age of evidence). b) carbon testing of other materials in the burial, such as charcoal or other organic burial materials. Usually pretty reliable, as long as the grave area was previously untouched. This is a combination of absolute and relative methods (we can identify the absolute age of charcoal, but we cannot be 100% that the charcoal was buried with the individual). c) relative dating is a common non-invasive method archeologists use. Essentially, if you know that X burial good was constructed 120 years ago, then it is likely that everything else on that substrate (level below ground) is relatively close in age. d) Additionally, the relative age of remains can be identified through other means, such as examining the remains for signs of medical treatment. Medical treatments change drastically over the centuries and can often indicate when an individual lived. This can range from dental practices, to examining the remains for differing types of trauma. For instance, if I'm excavating an individual with extreme dental issues with no signs of treatment, in combination with something such as mild break that was poorly healed, there's a good chance that the burial is quite old. But this should be used in conjunction with other methods where possible.

I am not sure how Canada will approach identifying mass graves NOT at these residential schools. They would first have to identify the remains (making sure that they are indigenous burials) and then they would likely proceed as outlined above (skipping step 1,as presumably the site has been located).

Hopefully that answers your questions.