How do I know if my radiator needs bleeding?

If the top of your radiator is noticeably cooler than the bottom, then this is good indicator that the radiator needs bleeding.  All you need is a bleed key and a cloth to soak up any water that may be released when you open the bleed valve.  If however you are finding that your radiators need to be regularly bled, then this indicates a potential leak somewhere in the heating system.  The services of a plumber would be recommended in this instance.