Go to the Ft Sam Cemetery page and after it opens, cluster pins come up in the photo window. This takes some time for Ft Sam and other very large veteran cemeteries.
After the pin clusters show up, you can scan around and find an area where there are gaps in that patterns (I just went to look and there are several obvious places).
Click on an area where the cluster pins are sparse and zoom in, until it shows the individual grave pins. It is slow and may bog down, with so many, but be patient and you will start to see holes in the patterns. Pick out a couple of areas, and make a note of where they are.
For Ft Sam, I would suggest that you try to find a map that you can print off, or print from Google maps, so you can mark the sections that are already done, or the ones you want to target.
Another thought is that some of the many dedicated folks who have already done photos there might have a working group in BG, so they keep track of what has been done and target their next forays. You might check with the local LDS stakes to see if someone is in a ft sam working group in BG.