The cap on a centre-spindle reservoir is held by a wingnut. The level marking is on the side of the reservoir.
Make sure the car is standing on flat ground. There may be hot and cold level marks inside the top of the reservoir.
If not, the level may be up to the top of a circular filter plate fitted to the centre spindle; look in the car handbook to find whether this is the hot or cold level.
On a centre-spindle reservoir, the whole lid is removed by unscrewing a wingnut.
Alternatively, there may be a dipstick on the bottom of the cap. Remove the cap, wipe the stick with a lint-free rag, screw back fully and remove again to read the level.
How to bleed power steering
Put on the handbrake and keep the car in neutral gear; if the car is an automatic, put it in Park before starting the engine.
Run the engine until it reaches normal working temperature. Leave it idling.
Turn the steering from lock to lock several times to heat the fluid. Switch off the engine.
Look into the reservoir; if there are bubbles, there is air in the system. Top up the fluid to the hot level and replace the cap.
Jack up the front of the car with both wheels just off the ground. Turn the steering from lock to lock three times.
Check the fluid level, topping up if necessary. Start the engine.
Slowly turn the wheels from lock to lock three times. Check the fluid level again, and top up if necessary. Note the exact level when you have done this. Replace the reservoir cap and switch off.
Lower the car and restart the engine. Turn the steering from lock to lock five times, then centre it exactly. Switch off, and look in the reservoir.
There should be no bubbling or frothing. The fluid level should not have risen by more than a small amount.
If the fluid is bubbling or has risen much, repeat the whole process from the start.
Finally re-examine the complete system for leaks. If there is still a problem, have the system checked by a garage.