As the Wulf says, stepping outside to check the weathger is a good awy to check the temperature.
Also as he says, you need two things to work.
A theme that has the temperature display icon available. Most modern themes will have this but check first before you buy or install.
Here's mine.
I use BerryWeather which you can get from here
BerryWeather - Personal Productivity BlackBerry Apps - Crackberry BlackBerry Apps Store
However you will also see from my screenshot that I also have the BerryWeather icon displayed on the Home screen and I see a lot more weather information than just the little thermometer which, I only put on for this screenshot.