Before checking the hydraulic oil level make sure that the mast on the forklift is fully lowered. If it is not there will be a lot of oil in the mast cylinders and when you check the level in the tank you will get an incorrect reading. Also make sure that the forklift is parked on flat level ground.
Some forklifts have a separate tank for the hydraulic oil and some use the box section which makes up the truck's chassis. In either case there will be a filler cap on the tank and very often the dipstick is attached to that as shown here. In other cases there might be a separate dipstick. Check the manufacturer's handbook if in doubt.
The hydraulic oil level should be to the manufacturers specification and using the correct type of oil to top up is vital. Do NOT top up the hydraulic oil level unless you are certain you are using the correct oil. Also make sure that no dirt is allowed to enter the tank as even microscopic amounts can cause serious damage to the system.