Many acoustic guitars will eventually have a brace come loose. The repair itself would be easy if not for having to address it through the sound hole and often using a mirror to see what is going on! As with most repairs that require re-gluing, the most important thing is that the joint is clean. Simple enough, but again, we are working in a space with very limited access.
So how do you know if you have a loose brace? Even looking inside the guitar, it may not be obvious. The give away is a rattling sound. If you notice this, you can often tap around the top and back and locate the loose brace.