You find this by using the Liquidity module combined with the billing schedule:
Set the project dates on the project — a billing schedule is then generated automatically, which can be adjusted to the actual planning.
Go to Liquidity (under Reporting) and choose Deviation from budget in the Numbers field.
There you see the difference between the planned and invoiced amount, both per month and cumulatively.
A positive deviation means the project is over-invoiced (more has been invoiced than the plan indicates), a negative deviation means under-invoiced.
Note: a single KPI figure on its own often gives a misleading picture, since a deviation can be due to timing just as much as to real differences in amounts. Assess the figure in the context of how far into the project you are.