The best thing I know is an old guitar that has not been repaired before. Then you can be sure that it is assembled as expected and with correct glues and varnish. Old repairs are seldom done right and sometimes in unexpected ways.
Some classics are that carpenter glue was used to repair cracks or glue the neck. Screws and nails are not uncommon, typically there may be screws in the bridge, through the neck foot or the fingerboard. Sometimes in other unexpected places, e.g. the screw through the neck foot may have been hidden behind the cover plate on the tip of the foot. Nails are less common but they occur. In worse cases, the guitar has been sanded down or sprayed on with a plastic varnish. There is really nothing you can be surprised about in an old repair. Sometimes, however, a repair is professionally done with the right glue and varnish, then it is mostly a violin maker who has done the job.
In the current batch, there are some that have been repaired before. The worst one is probably the one with the monster bridge. The sides are cracked from the neck block to the bottom block on both sides and the top has several cracks too. Everything glued with carpentry glue. The bottom had also been re-glued with carpentry glue, when I loosened the bottom I had to sacrifice the top of the neck block so as not to ruin the bottom. A neck block can be spliced without it being visible, a damaged bottom can not be hidden away.
Carpenter glue is a nuisance, but perhaps also the reason why the guitar still exists. But unlike epoxy glue, which is completely hopeless, there is a trick you can use. By soaking the glue joint with a wet kitchen paper for half an hour or so, the carpenter's glue can be saturated with moisture. Then the glue becomes white and clearly visible, also softer and can be scraped off with a knife or scrubbed away with a hard brush. You can also heat and soften the glue with a hot air gun or hair dryer, but it is important to be careful with lacquered surfaces.
I soak the glue on both sides if possible, the glue becomes white and easy to both see and remove. The long glue joint on both sides took some time to clean.
The same thing was done for some of the cracks in the top.
Here you can see how the glue changes color to white. With a sharp spatula and a knife you can push the tough glue from the crack, with a strong pig brush (preferably with dried hot hide glue) the last glue can be rubbed off.
With the wooden surfaces properly cleaned, you can then glue the cracks together with hot hide glue and various wooden patches and inlays.
The damaged neck block must be repaired with a piece of new spruce. With a simple jig and a dremel, I mill off the top layer of the neck block.
A fitting piece in spruce is glued on hot hide glue. The new piece can be easily planed down and the bottom gets a new flat surface to be glued to.