The biggest failures with a Spanish style guitar can usually be associated with the Bracing Daniel. So rather than having to perform ridiculous contortions trying to have a look, the easiest way of checking things out is do do some gentle tapping all over the Body, if there is a rogue Brace, you'll hear it as a definite thunk. Check everywhere, Top, Back and sides. If you hear a strange vibration when playing a note or chord, it's a sure sign that a glue joint is giving up.
Other than that..... Obviously sight down the neck, no Truss Rods here y'know, so if you see too much relief, walk away.