How to calculate denticity of ligands?

When looking at an ligand, how do I know its denticity? For instance, I know that though there are 2 lone pairs in $\ce{H2O}$, its denticity is only 1 due to geometrical reasons as the 2 of them can't be facing the metal ion at the same time. However, when I look at other ligands, especially unfamiliar ones, I cannot determine their denticity.


