I'm sorry how does the variance in surface temperature help us find it?(especially with current technology). As I understand it pretty much all the exoplanets so far have been found using Doppler shift. The size of the planet, more precisely the mass, and its distance from the star would influence the magnitude of the shift and hence detectability.
It doesn't help you find it. What it does is widen the acceptance criteria.
Instead of needing to find a rotating planet exactly in the "Goldilocks zone", any planet whose surface is hot on one side but cool on the other, such that the range of temperatures in some part of it lies in the zone, is acceptable, because you can suppose a zone of habitability on the edge of light and shadow.
I'm guessing that's easier because if the planet is a bit too hot on average, you can shift the zone into the shadow; and if it's too cool on average, you can shift the zone into the light. If the planet rotates, you've got a bigger problem, especially if it's too hot, because you need to find / build shade.
