How can you tell when a molecule has induced or permanent dipole dipole forces when looking at the molecules
All molecules have induced dipoles- it’s more a matter of determining whether there are also permanent dipoles or not.
In which case, you want to compare the electronegativities of the constituent atoms and consider whether the shape of the molecule would cancel any dipoles or not.
polar have permanent dipoles due to electronegativity different and you can determined this by if the molecules have an even electron distribution, e.g. if the bonding groups/molecules charges cancel out this would make it non polar if they did cancel and so not having permanent dipoles.