that’s not true. there are things that are not definitions. like my bed for instance, there are aspects that are arbitrary (my personal preferences, design choices, etc) and aspects that are not arbitrary (its physical form that exists beyond definition)
but you can’t! are you personally able to verify the results of every scientific investigation ever performed? think about what’s currently happening in psychology. loads of old foundational studies have been found to be irreproducible. and yet people had faith that they were conducted honestly and appropriately