How do I monitor CPU temperature on a 2009 MacBook Air Lion

My beloved 2009 MacBook AIr is freezing up. I think it is overheating. How can I monitor the CPU temp to see if this is the problem?

The Apple Store folk thought the SSD drive has gone bad, but they wanted $799.00 to replace it. I bought a new 2013 MacBook Air, but returned it because the screen had a purplish haze to it and the contrast was poor (to my eyes). I love my 2009 MacBook Air screen, so I want to fix it.