A cheap Infrared Thermometer is a tool i've found useful many times.
I got mine for $25 at radioshack (though i now see they cost more
there) and have seen them elsewhere. This should be an accurate way of
measuring temperature differences.

http://www.radioshack.com/sm-non-contact-infrared-thermometer--pi-2103171.html

http://www.amazon.com/exec/obidos/search-handle-url/index=blended&field-keywords=ir%20thermometer&results-process=default&dispatch=search/ref=pd_sl_aw_jset-1_med-u7internal-tops-misc_26249839_1&results-process=default?tag2=amd-google-20

-Craig
'85GL with overheating issues (turned out to be thermostat)