Sentence:  “[…] the awfully hearty sort of Christmas cards that people do send to other people that they don't know at all well. You know. The kind that have mottoes.... And then, when you see [the senders], you probably <WSD>find</WSD> that they are the most melancholy old folk with malignant diseases. […]” 
Meaning of the word marked with <WSD> tags:  To decide that, to discover that, to form the opinion that.