I'm no expert on Excalibur bows but I do know that when shooting most bows in either compound or recurve, the arrow/bolt absords about 70-75 percent of the released energy and dry firing is a definite NO-NO.
I can't see how the design of the Excalibur would be any less stressfull on their limbs versus any other recurve bow. Would like to know the answer to that one.
Could it be that Excalibur just has a warranty that will replace exploded/cracked limbs if someone dry fires their bows?
FWIW, if I bought an Excalibur, I wouldn't dry fire it regardless.
To the OP's question. Other then visually inspecting the limbs(even using a magnifying glass), cams etc. for stress cracks and seeing them, I don't know of another method for a person to 'spot check' a bow for damages. Then there may be stress cracks not visible to the naked eye.
Other then having the limbs x-ray'ed, I don't know of a guaranteed way checking the limbs.
Buying any used bow can be a gamble.