See exactly how the money's flowing — collected, outstanding, and trending — on the Analytics Business tab.
Who can do this: Requires the Analytics add-on. Turn it on from Add-ons.
On the dashboard
Open Analytics → Business:
Revenue this month — what you've billed, with the change versus last month.
Outstanding balance — what clients still owe, and how many invoices it spans.
Revenue trend and Outstanding aging — charts that show where revenue is heading and how old your unpaid invoices are.
Switch the time range (This Week through YTD) to see any window.
Without the add-on
Even without Analytics, your Hub shows this month's payments, and the Invoices tabs (Paid, Overdue) show collected and outstanding money. See What do the different invoice statuses mean?