A site called 'TipKing' advises
You can check your oven's temperature with this simple test. After preheating it, lay a sheet of plain white paper on the centre rack and leave it for five minutes. The paper's colour will tell you the oven's temperature:
* pale biscuit, 150°C/300°F or less
* light brown, 180°- 200°C/350°-400°F
* golden brown, 200°- 230°C/400°-450°F
* dark brown, 230°- 260°C/450°-500°F
* black, over 260°C/500°F
To determine the temperature in a wood-burning oven by the same method, let the fire burn for 20 minutes to preheat the oven.
Second option -
http://www.amazon.co....ilicone/dp/B000EW6ZOE
Salter Thermometer at Amazon £9.45
Third Option
http://cgi.ebay.co.uk...0a0e204148db4fd154b9c
Thermometer from E-bay for a fiver including PnP