Its usually easy to know if you need to bleed you radiators. When your heating is turned on, touch the radiator and have a feel. If it does require bleeding it will be colder towards the top of the radiator. This is caused by a build up of air which gathers at the top of radiator or in high level pipework which prevents the water flowing correctly.
If you have had your heating off for a long period of time you may not be able to feel any heat difference between the top and bottom of the radiator. Just compare all the radiators on your heating system and see which ones are cooler. The cooler ones may require bleeding the most. If all the radiators are hot except for the one you’re examining, and the radiator hasn't been turned off, then there is a may be problem and you’ll need to contact a local plumber.
How to bleed a radiator: the basics
1. Turn your heating system on and let it get warm. Once it is warm switch it off again.
2. You will then need a "bleed key", which can be bought in any good DIY stores. This is a tool which opens and closes the vent of your radiator. You may be able to use a screwdriver if there is a slot on your radiator vent.
3. Put the bleed key into the radiator vent which is found at the top of your radiator.
4. Make sure you have an old rag handy. Turn the bleed key slightly until you can hear the air "hissing" out.
5. Once the "hissing" stops and water comes out you’ll know that all the air has escaped. Now use the bleed key to tighten up the vent again.
6. Check all the radiators are free of air and then you can turn your heating back on. If you have a combi boiler make sure there is enough pressure in the boiler before switching it back on. You can see how to refill you combi boiler here