How do you know if the fan in a laptop is broken?

I agree. Laptop fans are very easy to hear. If you are running your computer and acually working on it for more than 20 min and you don't hear the fan kick on, there is definately something wrong. If you would like to check it out your self. Many computers have the ability to monitor these things. You can either download a system monitor program or look in your Bios Menu. You should have a system monitor in there where it should tell you the RPM's of the fan/fans if it is supported.