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It's best to do this when the car is stopped on a level surface, with the engine cold. On most newer cars, the reservoir is transparent, with lines marked "Min" and "Max"; the brake fluid level should fall between these lines. Cars older than the 1980s may have metal reservoirs, requiring you to remove the reservoir cap. (Newer caps screw on and off, while some older caps need to be pried off with a screwdriver.)
Open the car's hood. Check the fluid level in the reservoir.