check up on vs suss out

check up on

verb
  • To examine or inspect something in order to determine its condition; to check out. 

  • To look after or inquire about someone's well-being. 

suss out

verb
  • To manage to work (something) out, to determine (something). 

  • To come to understand (a person). 

How often have the words check up on and suss out occurred in a corpus of books? (source: Google Ngram Viewer )