it is kinda hard to mesure temp of smd leds on strip, if i had to i would use contact probe thermometer, (not ir thermometer), i would take readings right at the solder point of the led, (if the tape is not waterproof\silicone enveloped, if it is , i would open small section) and would turn your fan, when temp gets 70C. or i would get one of those thermal switches that are NO, and place it right under led, i would use 65c switch, you can splice it into power feed, of your fan, just make sure current rating of thermostat is not less of your circuit, or it'll close once, and will most likely get stock, and wont open anymore.
but to avoid all that, you can run strip at 9v instead of 12, and you wont have to worry about temp. i've done it may times like that, no heat issues, unlike driving them at 12v where i had many thermal issues.