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 your contacts list overview.

"Last activity" on contacts level 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
• create a project 
• create a meeting for a project for this 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 meeting for this contact that isn't linked to a project