check up on vs investigate

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. 

investigate

verb
  • To examine, look into, or scrutinize in order to discover something hidden or secret. 

  • To inquire into or study in order to ascertain facts or information. 

  • To conduct an inquiry or examination. 

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