To check the brake fluid in a car, locate the brake fluid reservoir under the hood. It is usually located near the firewall on the driver's side. The reservoir is typically clear or translucent, allowing you to see the level of the brake fluid. Make sure the car is level, then remove the cap and check the level of the fluid. The brake fluid should be between the minimum and maximum marks on the reservoir. If it is low, add the recommended type of brake fluid.