You need to determine which ...

You need to determine which parameter will be limiting. You will need to analyse for any potentially scaling parameter. In most waters these will include the alkali earth metal cations, calcium, magnesium and barium are the typical major ones, the associated anions, typically sulphate is the major one because the pH correction will take care of carbonates and hydroxides, and lastly silica. 

If you are treated recovered effluent, then the list could grow, although you would typically precipitate transitional metals prior to UF or NF.