Let's say you want to know if a contact in your directory has actually done a project with you before.

Note: this can be done from your directory or from within a project.

First, click the contact's name.

Then, click the Activity Tab.

From here, you can see all upcoming or past projects.
As you can imagine, if a contact has no activity, then their a lead. If a client has activity, then they are/were a client.