Using the experimental results, we have calculated the density distributions of valence nucleons in terms of the single-particle potential model. The outside probabilities and contributions of valence nucleons given by these calculations are considered as the criterion of halo states. By studying the variation of root-mean-square radius with the separation energy, conditions for the existence of halo states are pointed out, especially of the proton halo states. These have realistic meanings for judging and searching the halo states.