You Can't! It doesn't matter. See how many turns they are from being closed. A lockshield on a small radiator near the pump will be only a quarter a turn or so open whereas one the other side of the house on a big radiator will be much wider.
Better still practice is to invest in some TRV's! Then the opposite end to which you fit them will become the lockshield valve.
That way you could save some money
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local