If you want to check how much retainer has been used for a project and how much is remaining, follow these steps:

1. Open the Retainers screen from the side menu > Billing > Retainers.
2. From the Project drop-down, select the project whose details you want to check.
3. The project retainer used on a project shows up against the field Project Retainer Used and the retainer left shows up against Project Retainer Balance.

Check the Core Help for details.