Comparing bond lengths of a molecule using valence bond theory?

My textbook says that all the bonds of $\ce{SF4} $ are not equal while in the case of $\ce{XeF\_2}$, $ \ce{BF4-} $ or $\ce{SiF4} $ all the bonds are equal. No reason is stated for this observation. 


So is there a general way or formula to check if all the bond lengths are equal or not for a given molecule?


