How do I know if my MBA is overheating

I have a 6 month old MBA bought from an Apple store. It is running 10.7.3. I have noticed that the fan is coming on very often (i.e. several times a day) while doing things such as working in MS Word or browsing the Internet with Firefox. I do have several documents & browser windows minimized and in the dock, but nothing more intensive than that.

Is this normal?

If and when it does truly overheat it will shut down. Just before that happens it will get slow as the CPU will go into a throttling mode where it lowers the clock cycles to try and stay cooler.