Determining spin of metal complex
Are you asking experimentally or theoretically?
Theoretically speaking for general cases you should know what is the energy of the high-spin and low-spin configuration, and in given state, what is the approximate spin and vibrational entropy so you can estimate an equilibrium constant for given temperature. Now this either needs some calculation, or some sort of database from where you can make this estimations. 

Experience (i.e. database or whatever series) says that water is a weak ligand, from that you generally guess that water complexes are generally in high spin state. 

Experimentally you can determine spin state by measuring magnetic susceptibility, interpreting EPR, measuring 57Fe Mossbauer spectrum etc