the best and cheapest way would be to ask the salesperson or read the manual, what you are looking for is watts, 1 kw per hour = 1 unit, that is to say for example a 100w bulb burning for 10 hours = 1 unit, all devices will have wattage displayed somewhere on them, those plug in devices are not worth the money as only one product at a time, a simple calculation (basic maths as above)will give the same result. however if you are really worried about power consumption or carbon footprint i would strongly recommend an electrisave, not only can you see what each item uses in real time at the flick of a switch,as the plug in adapter does, but will give you real time power consumption for the whole household 24 7, it works, i have one, when you can see what cost 1p per hour to leave on standby you will switch it of, because 1p per hour = 24p per day, 96p for four days etc etc, the biggest advantage is the quick look before you go out, you can see if the iron, hair straighteners etc etc has been left on,
for me this device that seems expensive at the £70 price tag will save me about £200 this year alone.
http://www.electrisave.co.uk/
SAVE A PENNY PER HOUR = £86.70 PER YEAR = the meter has paid for itself and saved you some, and the fact you will switch of something you may have left on will be a big saving