If you want to see which contacts need some attention, you can check the 'last activity' date. 
It's possible to add this column to the overview of all your contacts (your contacts list), but on individual contact pages you'll have to scroll down to 'Activities' to see the history.

The column "Last activity" on the contact overview page updates when you:

• add a note to the activities on the contact page
• finish a call
• finish a task
• finish a meeting linked to this contact
• send an email or track a sent mail
• create an invoice
• send an invoice
• edit the background information
• add a timesheet
• create a subscription
• merge 2 contacts together (this edits the last activity for the remaining contact)
• deactivate the contact
• add a file to the contact

This field doesn't update however when you:

• create a deal for this contact
• create a call for this contact
• create a task for this contact
• create a project for this contact

! This behaviour works the same for the company overview page as well, with the only difference that the account manager field also changes the last activity date for companies.