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.