To establish the cycle of concentration for your installation, one needs a feedwater quality analysis. Regarding "...the best method.." it depends on the source of the feedwater. If served from your water utility, ask them or served from a river take a sample to the nearest lab.
Most water testing labs will offer a standard analysis which includes all the basics, for example, hardness, ph, temporary hardness, silca, iron, copper, turbidity, conductivity, etc. etc. If you need special tests, e.g. legionella that will cost extra.
cheers,