John:
Lay the guitar on it's back. Get a telescoping mirror with an articulating joint and a strong flashlight. Stick the mirror inside, shine light on the mirror then use your forearm and gently apply pressure in different areas of the top until you locate the loose brace. Keep a close look at the joint between the brace and top and watch for a small separation.
If you locate the loose brace it can be fixed with medium CA. You will have to be creative to position the guitar to get glue so it will run into the joint as you apply enough pressure to open the gap.
For clamping force you can make a small jack out of a turnbuckle and do some dry fits until you get the feel for how it feels as you will be installing the jack blind through the soundhole. I make little wooden feet for my jacks and tape them in place before gluing. Use tape between the wooden feet or wax paper so the jack doesn't become a permanent member of the guitar
You can also cut a large piece of mirror so it will fit through the sound hole and lay it on the guitars back. Take the bridge pins out and look down through the pin holes to see what you are doing inside the guitar. Lay a bright flashlight inside the guitar to illuminate the interior.