As Jedi Master said, there are "habitable zones" around stars. Depending on the size and intensity of the star, this region can grow or shrink, can be wider or thinner across. So, to be habitable, a planet must first fall into this zone. After that it comes to planetary make up, etc. Venus, Earth and Mars are within the Sun's habitable zone (as Jedi Master said) but only Earth has any life, at least overtly, because of the planet's atmosphere, hot liquid iron core which generates a magnetic field, and other factors as well (we don't have an acidic atmosphere, for example, like Venus).
So, if they can calculate the class of the star, they can predict its habitable zone, and if these planets fall out of those zones, then there is likely no chance they are habitable.
Wow, I actually remembered something from Astronomy!!!