A shunt is the easiest for this application but it is also easy to use
an Allegro hall-effect current sensing chip. Since you're only building
 one or two this may be a good way to go. The hall effect chips are
isolated so you can easily measure the current on the positive output of
the supply. The best part is that they'll probably send you free samples.
<http://www.allegromicro.com/en/Products/Part_Numbers/0715/index.asp>

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Martin
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